The switch is referred to as a "gate" when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i.e. a computer is a system of electronic switches which function as logical gates.
Three tactile switches. Major scale is inches.
A railroad switch is not electrical, but a mechanical device to divert a train from one track to another.
Contacts
A toggle switch in the "on" position.
In the simplest case, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to break the circuit. The contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working. Contact materials are also chosen on the basis of electrical conductivity, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity[1].
Sometimes the contacts are plated with noble metals. They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used.
Actuator
The moving part that applies the operating force to the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage (see photo).
Contact arrangements
Triple Pole Single Throw (TPST or 3PST) knife switch used to short the windings of a 3 phase wind turbine for braking purposes. Here the switch is shown in the open position.
A pair of contacts is said to be 'closed' when there is no space between them, allowing electricity to flow from one to the other. When the contacts are separated by an insulating air gap, an air space, they are said to be 'open', and no electricity can flow at typical voltages.
Switches can be and are classified according to the arrangement of their contacts in electronics fields— but electricians in the electrical wiring service business and their electrical supplier industries use different nomenclature, such as "one-way", "two-way", "three-way" and "four-way" switches—which have different meanings in North American and British cultural regions as is delineated in the table below.
Some contacts are normally open (Abbreviated "n.o." or "no") until closed by operation of the switch, while others are normally closed ("n.c. or "nc") and opened by the switch action, where the abbreviations given are commonly used on electronics diagrams for clarity of operation in assembly, analysis or troubleshooting. The serve to synchronize meaning with possible mistakes in wiring assembly, where wiring part of switch one way and part another (usually opposite) way will pretty much guarantee things won't work as designed.
A switch with both types of contact is called a changeover switch or "make-before-break" switch contact, whereas most switches have a spring loaded action which momentarily disconnect the load and so are "break-before-make" types by contrast— which type is used could be important, if for example, the switch selects two different power sources instead of switching circuit loads, or the circuit load will not and cannot tolerate any interruption in applied power.
The terms pole and throw are also used to describe switch contact variations. A pole is a set of contacts, the switch's electrical terminals that are connected to and belong to a single circuit, usually a load. A throw is one of two or more positions (the nomenclature is also applied to rotary switches, which can have many 'throw' positions) that the switch can adopt, which normally, but not always correspond to the number positions the switch handle or rotor can take when connecting between the common lead of the switch and a pole or poles. A throw position which connects no terminals (poles), has a mis-match between positions and positions which connect terminals, but are quite useful to turn things "Off" or for example, alternatively select between two scaled modes of operation. (e.g. Bright illumination, moderate illumination, no illumination.)
These terms give rise to abbreviations for the types of switch which are used in the electronics industry such as "single-pole, single-throw" (SPST) (the simplest type, "on or off") or "single-pole, double-throw" (SPDT), connecting either of two terminals to the common terminal. In electrical power wiring (i.e. House and building wiring by electricians) names generally involving the suffixed word "-way" are used; however, these terms differ between British and American English and the terms two way and three way are used in both with different meanings.
Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number or in some cases the letter "T" (for "triple"). In the rest of this article the terms SPST, SPDT and intermediate will be used to avoid the ambiguity in the use of the word "way".
Make-before-break, break-before-make
In a multi-throw switch, there are two possible transient behaviors as you move from one position to another. In some switch designs, the new contact is made before the old contact is broken. This is known as make-before-break, and ensures that the moving contact never sees an open circuit (also referred to as a shorting switch). The alternative is break-before-make, where the old contact is broken before the new one is made. This ensures that the two fixed contacts are never shorted to each other. Both types of design are in common use, for different applications.
Biased switches
A biased switch is one containing a spring that returns the actuator to a certain position. The "on-off" notation can be modified by placing parentheses around all positions other than the resting position. For example, an (on)-off-(on) switch can be switched on by moving the actuator in either direction away from the centre, but returns to the central off position when the actuator is released.
The momentary push-button switch is a type of biased switch. The most common type is a push-to-make switch, which makes contact when the button is pressed and breaks when the button is released. A push-to-break switch, on the other hand, breaks contact when the button is pressed and makes contact when it is released. An example of a push-to-break switch is a button used to release a door held open by an electromagnet. Changeover push button switches do exist but are even less common.
Special types
Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (the float switch), the turning of a key (key switch), linear or rotary movement (the limit switch or microswitch), or presence of a magnetic field (the reed switch).
Mercury tilt switch
The mercury switch consists of a drop of mercury inside a glass bulb with 2 contacts. The two contacts pass through the glass, and are connected by the mercury when the bulb is tilted to make the mercury roll on to them.
This type of switch performs much better than the ball tilt switch, as the liquid metal connection is unaffected by dirt, debris and oxidation, it wets the contacts ensuring a very low resistance bounce free connection, and movement and vibration do not produce a poor contact.
Knife switch
Knife switches are unique, because rather than employing an enclosed circuit connection area with a rubber- or plastic-insulated section for the user, the contacts and bridge are fully exposed.
The "knife", a flat metal swinging arm, is moved by the user between two or more contact areas. The knife and contacts are typically formed of copper, steel, or brass, depending on the application.
The primary advantage of a knife switch is the extremely high current capability inherent to the design. The amount of surface area on the "knife" that shorts the contacts is also extremely high, allowing a wide range of high voltage or high amperage applications with no circuit degradation, choke, or arcing during the switch throw. Thicker components need only be accompanied by wider contacts to conduct higher currents, which allows the design to scale extremely well with size.
Although knife switches are inferior to traditional switches in applications where user safety is paramount, knife switches are still commonly employed in everyday high-voltage applications such as building transformers, large power relays, and air-conditioning units.
NOTE:
2008年11月10日星期一
Keypad
A keypad is a set of buttons arranged in a block which usually bear digits and other symbols but not a complete set of alphabetical letters. If it mostly contains numbers then it can also be called a numeric keypad. Keypads are found on many alphanumeric keyboards and on other devices such as calculators, combination locks and telephones which require largely numeric input.
A telephone keypad
A computer keyboard usually contains a small numeric keypad with a calculator-style arrangement of buttons duplicating the numeric and arithmetic keys on the main keyboard to allow efficient entry of numerical data. This number pad (commonly abbreviated to "numpad") is usually positioned on the right side of the keyboard because most people are right-handed.
Many laptop computers have special function keys which turn part of the alphabetical keyboard into a numerical keypad as there is insufficient space to allow a separate keypad to be built into the laptop's chassis. Separate plug-in keypads can be purchased.
A calculator
By convention, the keys on calculator-style keypads are arranged such that 123 is on the bottom row. In contrast, a telephone keypad has the 123 keys at the top. It also has buttons labelled (star) and # (octothorpe, number sign, "pound" or "hash") either side of the zero. Most of the keys also bear letters which have had several auxiliary uses, such as remembering area codes or whole telephone numbers.
The keypad of a calculator contains the digits 0 through 9, together with the four arithmetic operations, the decimal point and other more advanced functions.
Keypads are a part of mobile phones that are replaceable and sit on a sensor board. Some multimedia mobile phones have a small joystick which has a cap to match the keypad.
Keypads are also a feature of some combination locks. This type of lock is often used on doors, such as that found at the main entrance to some offices.
NOTE:
A telephone keypad
A computer keyboard usually contains a small numeric keypad with a calculator-style arrangement of buttons duplicating the numeric and arithmetic keys on the main keyboard to allow efficient entry of numerical data. This number pad (commonly abbreviated to "numpad") is usually positioned on the right side of the keyboard because most people are right-handed.
Many laptop computers have special function keys which turn part of the alphabetical keyboard into a numerical keypad as there is insufficient space to allow a separate keypad to be built into the laptop's chassis. Separate plug-in keypads can be purchased.
A calculator
By convention, the keys on calculator-style keypads are arranged such that 123 is on the bottom row. In contrast, a telephone keypad has the 123 keys at the top. It also has buttons labelled (star) and # (octothorpe, number sign, "pound" or "hash") either side of the zero. Most of the keys also bear letters which have had several auxiliary uses, such as remembering area codes or whole telephone numbers.
The keypad of a calculator contains the digits 0 through 9, together with the four arithmetic operations, the decimal point and other more advanced functions.
Keypads are a part of mobile phones that are replaceable and sit on a sensor board. Some multimedia mobile phones have a small joystick which has a cap to match the keypad.
Keypads are also a feature of some combination locks. This type of lock is often used on doors, such as that found at the main entrance to some offices.
NOTE:
industrial mechanic components
active semiconductor components
2008年11月7日星期五
Word processor
OpenOffice.org Writer
A word processor (more formally known as document preparation system) is a computer application used for the production (including composition, editing, formatting, and possibly printing) of any sort of printable material.
Word processor may also refer to an obsolete type of stand-alone office machine, popular in the 1970s and 80s, combining the keyboard text-entry and printing functions of an electric typewriter with a dedicated computer for the editing of text. Although features and design varied between manufacturers and models, with new features added as technology advanced, word processors for several years usually featured a monochrome display and the ability to save documents on memory cards or diskettes. Later models introduced innovations such as spell-checking programs, increased formatting options, and dot-matrix printing. As the more versatile combination of a personal computer and separate printer became commonplace, the word processor disappeared.
Word processors are descended from early text formatting tools (sometimes called text justification tools, from their only real capability). Word processing was one of the earliest applications for the personal computer in office productivity.
Although early word processors used tag-based markup for document formatting, most modern word processors take advantage of a graphical user interface. Most are powerful systems consisting of one or more programs that can produce any arbitrary combination of images, graphics and text, the latter handled with type-setting capability.
Microsoft Word is the most widely used computer word processing system; Microsoft estimates over five hundred million people use the Office suite, which includes Word. There are also many other commercial word processing applications, such as WordPerfect, which dominated the market from the mid-1980s to early-1990s, particularly for machines running Microsoft's MS-DOS operating system. Open-source applications such as OpenOffice.org Writer and KWord are rapidly gaining in popularity.[citation needed] Online word processors such as Google Docs are a relatively new category.
Characteristics
Other word processing functions include "spell checking" (actually checks against wordlists), "grammar checking" (checks for what seem to be simple grammar errors), and a "thesaurus" function (finds words with similar or opposite meanings). In most languages grammar is very complex, so grammar checkers tend to be unreliable and also require a large amount of RAM.[citations needed] Other common features include collaborative editing, comments and annotations, support for images and diagrams and internal cross-referencing.
Word processors can be distinguished from several other, related forms of software:
Text editors (modern examples of which include Notepad, BBEdit, Kate, Gedit), were the precursors of word processors. While offering facilities for composing and editing text, they do not format documents. This can be done by batch document processing systems, starting with TJ-2 and RUNOFF and still available in such systems as LaTeX (as well as programs that implement the paged-media extensions to HTML and CSS). Text editors are now used mainly by programmers, website designers, and computer system administrators. They are also useful when fast startup times, small file sizes, editing speed and simplicity of operation are preferred over formatting.
Later desktop publishing programs were specifically designed to allow elaborate layout for publication, but often offered only limited support for editing. Typically, desktop publishing programs allowed users to import text that was written using a text editor or word processor.
Almost all word processors enable users to employ styles, which are used to automate consistent formatting of text body, titles, subtitles, highlighted text, and so on.
Styles greatly simplify managing the formatting of large documents, since changing a style automatically changes all text that the style has been applied to. Even in shorter documents styles can save a lot of time while formatting. However, most help files refer to styles as an 'advanced feature' of the word processor, which often discourages users from using styles regularly.
Document statistics
Most current word processors can calculate various statistics pertaining to a document. These usually include:
Character count, word count, sentence count, line count, paragraph count, page count. Word, sentence and paragraph length. Editing time.
Errors are common; for instance, a dash surrounded by spaces — like either of these — may be counted as a word.
Typical usage
Word processors have a variety of uses and applications within the business world, home, and education.
Businesses tend to have their own format and style for any of these. Thus, versatile word processors with layout editing and similar capabilities find widespread use in most businesses.
Education
Many schools have begun to teach typing and word processing to their students, starting as early as elementary school. Typically these skills are developed throughout secondary school in preparation for the business world. Undergraduate students typically spend many hours writing essays. Graduate and doctoral students continue this trend, as well as creating works for research and publication.
Home
While many homes have word processors on their computers, word processing in the home tends to be educational or business related, dealing with assignments or work being completed at home. Some use word processors for letter writing, résumé creation, and card creation. However, many of these home publishing processes have been taken over by desktop publishing programs specifically oriented toward home use such as The Print Shop, which is better suited for these types of documents.
History
The term word processing was invented by IBM in the late 1960s. By 1971 it was recognized by the New York Times as a "buzz word".[1] A 1971 Times article referred to "the brave new world of Word Processing or W/P. That's International Business Machines talk... I.B.M. introduced W/P about five years ago for its Magnetic Selectric typewriter and other electronic razzle-dazzle."[2]
IBM defined the term in a broad and vague way as "the combination of people, procedures, and equipment which transforms ideas into printed communications," and originally used it to include dictating machines and ordinary, manually-operated Selectric typewriters.[3] By the early seventies, however, the term was generally understood to mean semiautomated typewriters affording at least some form of electronic editing and correction, and the ability to produce perfect "originals." Thus, the Times headlined a 1974 Xerox product as a "speedier electronic typewriter", but went on to describe the product, which had no screen[4], as "a word processor rather than strictly a typewriter, in that it stores copy on magnetic tape or magnetic cards for retyping, corrections, and subsequent printout."[5]
Electromechanical paper-tape-based equipment such as the Friden Flexowriter had long been available; the Flexowriter allowed for operations such as repetitive typing of form letters (with a pause for the operator to manually type in the variable information)[6], and when equipped with an auxiliary reader, could perform an early version of "mail merge". Circa 1970 it began to be feasible to apply electronic computers to office automation tasks. IBM's Mag Tape Selectric Typewriter (MTST) and later Mag Card Selectric (MCST) were early devices of this kind, which allowed editing, simple revision, and repetitive typing, with a one-line display for editing single lines.[7]
The New York Times, reporting on a 1971 business equipment trade show, saidThe "buzz word" for this year's show was "word processing," or the use of electronic equipment, such as typewriters; procedures and trained personnel to maximize office efficiency. At the IBM exhibition a girl [sic] typed on an electronic typewriter. The copy was received on a magnetic tape cassette which accepted corrections, deletions, and additions and then produced a perfect letter for the boss's signature.... [1]
In 1971, a third of all working women in the United States were secretaries, and they could see that word processing would have an impact on their careers. Some manufacturers, according to a Times article, urged that "the concept of 'word processing' could be the answer to Women's Lib advocates' prayers. Word processing will replace the 'traditional' secretary and give women new administrative roles in business and industry."[1]
The 1970s word processing concept did not refer merely to equipment, but, explicitly, to the use of equipment for "breaking down secretarial labor into distinct components, with some staff members handling typing exclusively while others supply administrative support. A typical operation would leave most executives without private secretaries. Instead one secretary would perform various administrative tasks for three or more secretaries."[8] A 1971 article said that "Some [secretaries] see W/P as a career ladder into management; others see it as a dead-end into the automated ghetto; others predict it will lead straight to the picket line." The National Secretaries Association, which defined secretaries as people who "can assume responsibility without direct supervision," feared that W/P would transform secretaries into "space-age typing pools." The article considered only the organizational changes resulting from secretaries operating word processors rather than typewriters; the possibility that word processors might result in managers creating documents without the intervention of secretaries was not considered—not surprising in an era when few but secretaries possessed keyboarding skills.[2]
In the early 1970s, computer scientist Harold Koplow was hired by Wang Laboratories to program calculators. One of his programs permitted a Wang calculator to interface with an IBM Selectric typewriter, which was at the time used to calculate and print the paperwork for auto sales.
In 1974, Koplow's interface program was developed into the Wang 1200 Word Processor, an IBM Selectric-based text-storage device. The operator of this machine typed text on a conventional IBM Selectric; when the Return key was pressed, the line of text was stored on a cassette tape. One cassette held roughly 20 pages of text, and could be "played back" (i.e., the text retrieved) by printing the contents on continuous-form paper in the 1200 typewriter's "print" mode. The stored text could also be edited, using keys on a simple, six-key array. Basic editing functions included Insert, Delete, Skip (character, line), and so on.
The labor and cost savings of this device were immediate, and remarkable: pages of text no longer had to be retyped to correct simple errors, and projects could be worked on, stored, and then retrieved for use later on. The rudimentary Wang 1200 machine was the precursor of the Wang Office Information System (OIS), introduced in 1976, whose CRT-based system was a major breakthrough in word processing technology. It displayed text on a CRT screen, and incorporated virtually every fundamental characteristic of word processors as we know them today. It was a true office machine, affordable by organizations such as medium-sized law firms, and easily learned and operated by secretarial staff.
The Wang was not the first CRT-based machine nor were all of its innovations unique to Wang. In the early 1970s Linolex, Lexitron and Vydec introduced pioneering word-processing systems with CRT display editing. A Canadian electronics company, Automatic Electronic Systems, had introduced a product with similarities to Wang's product in 1973, but went into bankruptcy a year later. In 1976, refinanced by the Canada Development Corporation, it returned to operation as AES Data, and went on to successfully market its brand of word processors worldwide until its demise in the mid-1980s. Its first office product, the AES-90[9], combined for the first time a CRT-screen, a floppy-disk and a microprocessor,[citation needed] that is, the very same winning combination that would be used by IBM for its PC seven years later.[citation needed] The AES-90 software was able to handle French and English typing from the start. The first eight units were delivered to the office of the then Prime Minister, Pierre-Elliot Trudeau, in February 1974.[citation needed] Despite these predecessors, Wang's product was a standout, and by 1978 it had sold more of these systems than any other vendor.[10]
The phrase "word processor" rapidly came to refer to CRT-based machines similar to Wang's. Numerous machines of this kind emerged, typically marketed by traditional office-equipment companies such as IBM, Lanier (marketing AES Data machines, re-badged), CPT, and NBI.[11] All were specialized, dedicated, proprietary systems, with prices in the $10,000 ballpark. Cheap general-purpose computers were still the domain of hobbyists.
Some of the earliest CRT-based machines used cassette tapes for removable-memory storage until floppy diskettes became available for this purpose - first the 8-inch floppy, then the 5-1/4-inch (drives by Shugart Associates and diskettes by Dysan).
Printing of documents was initially accomplished using IBM Selectric typewriters modified for ASCII-character input. These were later replaced by application-specific daisy wheel printers (Diablo, which became a Xerox company, and Qume -- both now defunct.) For quicker "draft" printing, dot-matrix line printers were optional alternatives with some word processors.
With the rise of personal computers, and in particular the IBM PC and PC compatibles, software-based word processors running on general-purpose commodity hardware gradually displaced dedicated word processors, and the term came to refer to software rather than hardware. Some programs were modeled after particular dedicated WP hardware. MultiMate, for example, was written for an insurance company that had hundreds of typists using Wang systems, and spread from there to other Wang customers. To adapt to the smaller PC keyboard, MultiMate used stick-on labels and a large plastic clip-on template to remind users of its dozens of Wang-like functions, using the shift, alt and ctrl keys with the 10 IBM function keys and many of the alphabet keys.
Other early word-processing software required users to memorize semi-mnemonic key combinations rather than pressing keys labelled "copy" or "bold." (In fact, many early PCs lacked cursor keys; WordStar famously used the E-S-D-X-centered "diamond" for cursor navigation, and modern vi-like editors encourage use of hjkl for navigation.) However, the price differences between dedicated word processors and general-purpose PCs, and the value added to the latter by software such as VisiCalc, were so compelling that personal computers and word processing software soon became serious competition for the dedicated machines. Word Perfect, XyWrite, Microsoft Word and dozens of other word processing software brands competed in the 1980s. Development of higher-resolution monitors allowed them to provide limited WYSIWYG - What You See Is What You Get, to the extent that typographical features like bold and italics, indentation, justification and margins were approximated on screen.
A Brother WP-1400D hardware-type word processor from 1994.
The mid-to-late 1980s saw the spread of laser printers, a "typographic" approach to word processing, and of true WYSIWYG bitmap displays with multiple fonts (pioneered by the Xerox Alto computer and Bravo word processing program), Postscript, and graphical user interfaces (another Xerox PARC innovation, with the Gypsy word processor which was commercialised in the Xerox Star product range). Standalone word processors adapted by getting smaller and replacing their CRTs with small character-oriented LCD displays. Some models also had computer-like features such as floppy disk drives and the ability to output to an external printer. They also got a name change, now being called "electronic typewriters" and typically occupying a lower end of the market, selling for under $200 USD.
MacWrite, Microsoft Word and other word processing programs for the bit-mapped Apple Macintosh screen, introduced in 1984, were probably the first true WYSIWYG word processors to become known to many people until the introduction of Microsoft Windows. Dedicated word processors eventually became museum pieces.
NOTE:
A word processor (more formally known as document preparation system) is a computer application used for the production (including composition, editing, formatting, and possibly printing) of any sort of printable material.
Word processor may also refer to an obsolete type of stand-alone office machine, popular in the 1970s and 80s, combining the keyboard text-entry and printing functions of an electric typewriter with a dedicated computer for the editing of text. Although features and design varied between manufacturers and models, with new features added as technology advanced, word processors for several years usually featured a monochrome display and the ability to save documents on memory cards or diskettes. Later models introduced innovations such as spell-checking programs, increased formatting options, and dot-matrix printing. As the more versatile combination of a personal computer and separate printer became commonplace, the word processor disappeared.
Word processors are descended from early text formatting tools (sometimes called text justification tools, from their only real capability). Word processing was one of the earliest applications for the personal computer in office productivity.
Although early word processors used tag-based markup for document formatting, most modern word processors take advantage of a graphical user interface. Most are powerful systems consisting of one or more programs that can produce any arbitrary combination of images, graphics and text, the latter handled with type-setting capability.
Microsoft Word is the most widely used computer word processing system; Microsoft estimates over five hundred million people use the Office suite, which includes Word. There are also many other commercial word processing applications, such as WordPerfect, which dominated the market from the mid-1980s to early-1990s, particularly for machines running Microsoft's MS-DOS operating system. Open-source applications such as OpenOffice.org Writer and KWord are rapidly gaining in popularity.[citation needed] Online word processors such as Google Docs are a relatively new category.
Characteristics
Other word processing functions include "spell checking" (actually checks against wordlists), "grammar checking" (checks for what seem to be simple grammar errors), and a "thesaurus" function (finds words with similar or opposite meanings). In most languages grammar is very complex, so grammar checkers tend to be unreliable and also require a large amount of RAM.[citations needed] Other common features include collaborative editing, comments and annotations, support for images and diagrams and internal cross-referencing.
Word processors can be distinguished from several other, related forms of software:
Text editors (modern examples of which include Notepad, BBEdit, Kate, Gedit), were the precursors of word processors. While offering facilities for composing and editing text, they do not format documents. This can be done by batch document processing systems, starting with TJ-2 and RUNOFF and still available in such systems as LaTeX (as well as programs that implement the paged-media extensions to HTML and CSS). Text editors are now used mainly by programmers, website designers, and computer system administrators. They are also useful when fast startup times, small file sizes, editing speed and simplicity of operation are preferred over formatting.
Later desktop publishing programs were specifically designed to allow elaborate layout for publication, but often offered only limited support for editing. Typically, desktop publishing programs allowed users to import text that was written using a text editor or word processor.
Almost all word processors enable users to employ styles, which are used to automate consistent formatting of text body, titles, subtitles, highlighted text, and so on.
Styles greatly simplify managing the formatting of large documents, since changing a style automatically changes all text that the style has been applied to. Even in shorter documents styles can save a lot of time while formatting. However, most help files refer to styles as an 'advanced feature' of the word processor, which often discourages users from using styles regularly.
Document statistics
Most current word processors can calculate various statistics pertaining to a document. These usually include:
Character count, word count, sentence count, line count, paragraph count, page count. Word, sentence and paragraph length. Editing time.
Errors are common; for instance, a dash surrounded by spaces — like either of these — may be counted as a word.
Typical usage
Word processors have a variety of uses and applications within the business world, home, and education.
Businesses tend to have their own format and style for any of these. Thus, versatile word processors with layout editing and similar capabilities find widespread use in most businesses.
Education
Many schools have begun to teach typing and word processing to their students, starting as early as elementary school. Typically these skills are developed throughout secondary school in preparation for the business world. Undergraduate students typically spend many hours writing essays. Graduate and doctoral students continue this trend, as well as creating works for research and publication.
Home
While many homes have word processors on their computers, word processing in the home tends to be educational or business related, dealing with assignments or work being completed at home. Some use word processors for letter writing, résumé creation, and card creation. However, many of these home publishing processes have been taken over by desktop publishing programs specifically oriented toward home use such as The Print Shop, which is better suited for these types of documents.
History
The term word processing was invented by IBM in the late 1960s. By 1971 it was recognized by the New York Times as a "buzz word".[1] A 1971 Times article referred to "the brave new world of Word Processing or W/P. That's International Business Machines talk... I.B.M. introduced W/P about five years ago for its Magnetic Selectric typewriter and other electronic razzle-dazzle."[2]
IBM defined the term in a broad and vague way as "the combination of people, procedures, and equipment which transforms ideas into printed communications," and originally used it to include dictating machines and ordinary, manually-operated Selectric typewriters.[3] By the early seventies, however, the term was generally understood to mean semiautomated typewriters affording at least some form of electronic editing and correction, and the ability to produce perfect "originals." Thus, the Times headlined a 1974 Xerox product as a "speedier electronic typewriter", but went on to describe the product, which had no screen[4], as "a word processor rather than strictly a typewriter, in that it stores copy on magnetic tape or magnetic cards for retyping, corrections, and subsequent printout."[5]
Electromechanical paper-tape-based equipment such as the Friden Flexowriter had long been available; the Flexowriter allowed for operations such as repetitive typing of form letters (with a pause for the operator to manually type in the variable information)[6], and when equipped with an auxiliary reader, could perform an early version of "mail merge". Circa 1970 it began to be feasible to apply electronic computers to office automation tasks. IBM's Mag Tape Selectric Typewriter (MTST) and later Mag Card Selectric (MCST) were early devices of this kind, which allowed editing, simple revision, and repetitive typing, with a one-line display for editing single lines.[7]
The New York Times, reporting on a 1971 business equipment trade show, saidThe "buzz word" for this year's show was "word processing," or the use of electronic equipment, such as typewriters; procedures and trained personnel to maximize office efficiency. At the IBM exhibition a girl [sic] typed on an electronic typewriter. The copy was received on a magnetic tape cassette which accepted corrections, deletions, and additions and then produced a perfect letter for the boss's signature.... [1]
In 1971, a third of all working women in the United States were secretaries, and they could see that word processing would have an impact on their careers. Some manufacturers, according to a Times article, urged that "the concept of 'word processing' could be the answer to Women's Lib advocates' prayers. Word processing will replace the 'traditional' secretary and give women new administrative roles in business and industry."[1]
The 1970s word processing concept did not refer merely to equipment, but, explicitly, to the use of equipment for "breaking down secretarial labor into distinct components, with some staff members handling typing exclusively while others supply administrative support. A typical operation would leave most executives without private secretaries. Instead one secretary would perform various administrative tasks for three or more secretaries."[8] A 1971 article said that "Some [secretaries] see W/P as a career ladder into management; others see it as a dead-end into the automated ghetto; others predict it will lead straight to the picket line." The National Secretaries Association, which defined secretaries as people who "can assume responsibility without direct supervision," feared that W/P would transform secretaries into "space-age typing pools." The article considered only the organizational changes resulting from secretaries operating word processors rather than typewriters; the possibility that word processors might result in managers creating documents without the intervention of secretaries was not considered—not surprising in an era when few but secretaries possessed keyboarding skills.[2]
In the early 1970s, computer scientist Harold Koplow was hired by Wang Laboratories to program calculators. One of his programs permitted a Wang calculator to interface with an IBM Selectric typewriter, which was at the time used to calculate and print the paperwork for auto sales.
In 1974, Koplow's interface program was developed into the Wang 1200 Word Processor, an IBM Selectric-based text-storage device. The operator of this machine typed text on a conventional IBM Selectric; when the Return key was pressed, the line of text was stored on a cassette tape. One cassette held roughly 20 pages of text, and could be "played back" (i.e., the text retrieved) by printing the contents on continuous-form paper in the 1200 typewriter's "print" mode. The stored text could also be edited, using keys on a simple, six-key array. Basic editing functions included Insert, Delete, Skip (character, line), and so on.
The labor and cost savings of this device were immediate, and remarkable: pages of text no longer had to be retyped to correct simple errors, and projects could be worked on, stored, and then retrieved for use later on. The rudimentary Wang 1200 machine was the precursor of the Wang Office Information System (OIS), introduced in 1976, whose CRT-based system was a major breakthrough in word processing technology. It displayed text on a CRT screen, and incorporated virtually every fundamental characteristic of word processors as we know them today. It was a true office machine, affordable by organizations such as medium-sized law firms, and easily learned and operated by secretarial staff.
The Wang was not the first CRT-based machine nor were all of its innovations unique to Wang. In the early 1970s Linolex, Lexitron and Vydec introduced pioneering word-processing systems with CRT display editing. A Canadian electronics company, Automatic Electronic Systems, had introduced a product with similarities to Wang's product in 1973, but went into bankruptcy a year later. In 1976, refinanced by the Canada Development Corporation, it returned to operation as AES Data, and went on to successfully market its brand of word processors worldwide until its demise in the mid-1980s. Its first office product, the AES-90[9], combined for the first time a CRT-screen, a floppy-disk and a microprocessor,[citation needed] that is, the very same winning combination that would be used by IBM for its PC seven years later.[citation needed] The AES-90 software was able to handle French and English typing from the start. The first eight units were delivered to the office of the then Prime Minister, Pierre-Elliot Trudeau, in February 1974.[citation needed] Despite these predecessors, Wang's product was a standout, and by 1978 it had sold more of these systems than any other vendor.[10]
The phrase "word processor" rapidly came to refer to CRT-based machines similar to Wang's. Numerous machines of this kind emerged, typically marketed by traditional office-equipment companies such as IBM, Lanier (marketing AES Data machines, re-badged), CPT, and NBI.[11] All were specialized, dedicated, proprietary systems, with prices in the $10,000 ballpark. Cheap general-purpose computers were still the domain of hobbyists.
Some of the earliest CRT-based machines used cassette tapes for removable-memory storage until floppy diskettes became available for this purpose - first the 8-inch floppy, then the 5-1/4-inch (drives by Shugart Associates and diskettes by Dysan).
Printing of documents was initially accomplished using IBM Selectric typewriters modified for ASCII-character input. These were later replaced by application-specific daisy wheel printers (Diablo, which became a Xerox company, and Qume -- both now defunct.) For quicker "draft" printing, dot-matrix line printers were optional alternatives with some word processors.
With the rise of personal computers, and in particular the IBM PC and PC compatibles, software-based word processors running on general-purpose commodity hardware gradually displaced dedicated word processors, and the term came to refer to software rather than hardware. Some programs were modeled after particular dedicated WP hardware. MultiMate, for example, was written for an insurance company that had hundreds of typists using Wang systems, and spread from there to other Wang customers. To adapt to the smaller PC keyboard, MultiMate used stick-on labels and a large plastic clip-on template to remind users of its dozens of Wang-like functions, using the shift, alt and ctrl keys with the 10 IBM function keys and many of the alphabet keys.
Other early word-processing software required users to memorize semi-mnemonic key combinations rather than pressing keys labelled "copy" or "bold." (In fact, many early PCs lacked cursor keys; WordStar famously used the E-S-D-X-centered "diamond" for cursor navigation, and modern vi-like editors encourage use of hjkl for navigation.) However, the price differences between dedicated word processors and general-purpose PCs, and the value added to the latter by software such as VisiCalc, were so compelling that personal computers and word processing software soon became serious competition for the dedicated machines. Word Perfect, XyWrite, Microsoft Word and dozens of other word processing software brands competed in the 1980s. Development of higher-resolution monitors allowed them to provide limited WYSIWYG - What You See Is What You Get, to the extent that typographical features like bold and italics, indentation, justification and margins were approximated on screen.
A Brother WP-1400D hardware-type word processor from 1994.
The mid-to-late 1980s saw the spread of laser printers, a "typographic" approach to word processing, and of true WYSIWYG bitmap displays with multiple fonts (pioneered by the Xerox Alto computer and Bravo word processing program), Postscript, and graphical user interfaces (another Xerox PARC innovation, with the Gypsy word processor which was commercialised in the Xerox Star product range). Standalone word processors adapted by getting smaller and replacing their CRTs with small character-oriented LCD displays. Some models also had computer-like features such as floppy disk drives and the ability to output to an external printer. They also got a name change, now being called "electronic typewriters" and typically occupying a lower end of the market, selling for under $200 USD.
MacWrite, Microsoft Word and other word processing programs for the bit-mapped Apple Macintosh screen, introduced in 1984, were probably the first true WYSIWYG word processors to become known to many people until the introduction of Microsoft Windows. Dedicated word processors eventually became museum pieces.
NOTE:
Futuristic Computer Workstation
Application software
Application software is a subclass of computer software that employs the capabilities of a computer directly and thoroughly to a task that the user wishes to perform. This should be contrasted with system software which is involved in integrating a computer's various capabilities, but typically does not directly apply them in the performance of tasks that benefit the user. In this context the term application refers to both the application software and its implementation.[1]
A simple, if imperfect analogy in the world of hardware would be the relationship of an electric light bulb (an application) to an electric power generation plant (a system). The power plant merely generates electricity, not itself of any real use until harnessed to an application like the electric light that performs a service that benefits the user.
Typical examples of software applications are word processors, spreadsheets, and media players.
Multiple applications bundled together as a package are sometimes referred to as an application suite. Microsoft Office, OpenOffice.org, and iWork 08, which bundle together a word processor, a spreadsheet, and several other discrete applications, are typical examples. The separate applications in a suite usually have a user interface that has some commonality making it easier for the user to learn and use each application. And often they may have some capability to interact with each other in ways beneficial to the user. For example, a spreadsheet might be able to be embedded in a word processor document even though it had been created in the separate spreadsheet application.
User-written software tailors systems to meet the user's specific needs. User-written software include spreadsheet templates, word processor macros, scientific simulations, graphics and animation scripts. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is.
In some types of embedded systems, the application software and the operating system software may be indistinguishable to the user, as in the case of software used to control a VCR, DVD player or microwave oven.
It is important to note that this definition may exclude some applications that may exist on some computers in large organizations. For an alternate definition of an application: see Application Portfolio Management.
Terminology
The exact delineation between system software such as operating systems and application software is not precise, however, and is occasionally subject to controversy. For example, one of the key questions in the United States v. Microsoft antitrust trial was whether Microsoft's Internet Explorer web browser was part of its Windows operating system or a separable piece of application software. As another example, the GNU/Linux naming controversy is, in part, due to disagreement about the relationship between the Linux kernel and the Linux operating system.
In computer science, an application is a computer program designed to help people perform a certain type of work. An application thus differs from an operating system (which runs a computer), a utility (which performs maintenance or general-purpose chores), and a programming language (with which computer programs are created). Depending on the work for which it was designed, an application can manipulate text, numbers, graphics, or a combination of these elements. Some application packages offer considerable computing power by focusing on a single task, such as word processing; others, called integrated software, offer somewhat less power but include several applications. [2]
Application software classification
There are many subtypes of application software:
Enterprise software addresses the needs of organization processes and data flow, often in a large distributed environment. (Examples include Financial, Customer Relationship Management, and Supply Chain Management). Note that Departmental Software is a sub-type of Enterprise Software with a focus on smaller organizations or groups within a large organization. (Examples include Travel Expense Management, and IT Helpdesk)
Enterprise infrastructure software provides common capabilities needed to support Enterprise Software systems. (Examples include Databases, Email servers, and Network and Security Management)
Information worker software addresses the needs of individuals to create and manage information, often for individual projects within a department, in contrast to enterprise management. Examples include time management, resource management, documentation tools, analytical, and collaborative. Word processors, spreadsheets, email and blog clients, personal information system, and individual media editors may aid in multiple information worker tasks.
Content access software is software used primarily to access content without editing, but may include software that allows for content editing. Such software addresses the needs of individuals and groups to consume digital entertainment and published digital content. (Examples include Media Players, Web Browsers, Help browsers, and Games)
Educational software is related to content access software, but has distinct requirements for delivering evaluations (tests) and tracking progress through material. It is also related to collaboration software in that many Educational Software systems include collaborative capabilities.
Simulation software are computer software for simulation of physical or abstract systems for either research, training or entertainment purposes.
Media development software addresses the needs of individuals who generate print and electronic media for others to consume, most often in a commercial or educational setting. This includes Graphic Art software, Desktop Publishing software, Multimedia Development software, HTML editors, Digital Animation editors, Digital Audio and Video composition, and many others.
Product engineering software is used in developing hardware and software products. This includes computer aided design (CAD), computer aided engineering (CAE), computer language editing and compiling tools, Integrated Development Environments, and Application Programmer Interfaces.
NOTE:
A simple, if imperfect analogy in the world of hardware would be the relationship of an electric light bulb (an application) to an electric power generation plant (a system). The power plant merely generates electricity, not itself of any real use until harnessed to an application like the electric light that performs a service that benefits the user.
Typical examples of software applications are word processors, spreadsheets, and media players.
Multiple applications bundled together as a package are sometimes referred to as an application suite. Microsoft Office, OpenOffice.org, and iWork 08, which bundle together a word processor, a spreadsheet, and several other discrete applications, are typical examples. The separate applications in a suite usually have a user interface that has some commonality making it easier for the user to learn and use each application. And often they may have some capability to interact with each other in ways beneficial to the user. For example, a spreadsheet might be able to be embedded in a word processor document even though it had been created in the separate spreadsheet application.
User-written software tailors systems to meet the user's specific needs. User-written software include spreadsheet templates, word processor macros, scientific simulations, graphics and animation scripts. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is.
In some types of embedded systems, the application software and the operating system software may be indistinguishable to the user, as in the case of software used to control a VCR, DVD player or microwave oven.
It is important to note that this definition may exclude some applications that may exist on some computers in large organizations. For an alternate definition of an application: see Application Portfolio Management.
Terminology
The exact delineation between system software such as operating systems and application software is not precise, however, and is occasionally subject to controversy. For example, one of the key questions in the United States v. Microsoft antitrust trial was whether Microsoft's Internet Explorer web browser was part of its Windows operating system or a separable piece of application software. As another example, the GNU/Linux naming controversy is, in part, due to disagreement about the relationship between the Linux kernel and the Linux operating system.
In computer science, an application is a computer program designed to help people perform a certain type of work. An application thus differs from an operating system (which runs a computer), a utility (which performs maintenance or general-purpose chores), and a programming language (with which computer programs are created). Depending on the work for which it was designed, an application can manipulate text, numbers, graphics, or a combination of these elements. Some application packages offer considerable computing power by focusing on a single task, such as word processing; others, called integrated software, offer somewhat less power but include several applications. [2]
Application software classification
There are many subtypes of application software:
Enterprise software addresses the needs of organization processes and data flow, often in a large distributed environment. (Examples include Financial, Customer Relationship Management, and Supply Chain Management). Note that Departmental Software is a sub-type of Enterprise Software with a focus on smaller organizations or groups within a large organization. (Examples include Travel Expense Management, and IT Helpdesk)
Enterprise infrastructure software provides common capabilities needed to support Enterprise Software systems. (Examples include Databases, Email servers, and Network and Security Management)
Information worker software addresses the needs of individuals to create and manage information, often for individual projects within a department, in contrast to enterprise management. Examples include time management, resource management, documentation tools, analytical, and collaborative. Word processors, spreadsheets, email and blog clients, personal information system, and individual media editors may aid in multiple information worker tasks.
Content access software is software used primarily to access content without editing, but may include software that allows for content editing. Such software addresses the needs of individuals and groups to consume digital entertainment and published digital content. (Examples include Media Players, Web Browsers, Help browsers, and Games)
Educational software is related to content access software, but has distinct requirements for delivering evaluations (tests) and tracking progress through material. It is also related to collaboration software in that many Educational Software systems include collaborative capabilities.
Simulation software are computer software for simulation of physical or abstract systems for either research, training or entertainment purposes.
Media development software addresses the needs of individuals who generate print and electronic media for others to consume, most often in a commercial or educational setting. This includes Graphic Art software, Desktop Publishing software, Multimedia Development software, HTML editors, Digital Animation editors, Digital Audio and Video composition, and many others.
Product engineering software is used in developing hardware and software products. This includes computer aided design (CAD), computer aided engineering (CAE), computer language editing and compiling tools, Integrated Development Environments, and Application Programmer Interfaces.
NOTE:
Materials Procurement Software
Maritime Inspection Certification
Computer software
"Software" redirects here. For other uses, see Software (disambiguation).
Computer software, or just software is a general term used to describe a collection of computer programs, procedures and documentation that perform some tasks on a computer system.
A screenshot of the OpenOffice.org Writer desktop software
The term includes:Application software such as word processors which perform productive tasks for users, System software such as operating systems, which interface with hardware to provide the necessary services for application software, and Middleware which controls and co-ordinates distributed systems.
Software includes websites, programs, video games etc. that are coded by programming languages like C, C++, etc.
Firmware which is software programmed resident to electrically programmable memory devices on board mainboards or other types of integrated hardware carriers
Testware which is an umbrella term or container term for all utilities and application software that serve in combination for testing a software package but not necessarily may optionally contribute to operational purposes. As such, testware is not a standing configuration but merely a working environment for application software or subsets thereof.
"Software" is sometimes used in a broader context to mean anything which is not hardware but which is used with hardware, such as film, tapes and records.
Overview
Computer software is often regarded as anything but hardware, meaning that the "hard" are the parts that are tangible (able to hold) while the "soft" part is the intangible objects inside the computer. Software encompasses an extremely wide array of products and technologies developed using different techniques like programming languages, scripting languages etc. The types of software include web pages developed by technologies like HTML, PHP, Perl, JSP, ASP.NET, XML, and desktop applications like Microsoft Word, OpenOffice developed by technologies like C, C++, Java, C#, etc. Software usually runs on an underlying operating system (which is a software also) like Microsoft Windows, Linux (running GNOME and KDE), Sun Solaris etc. Software also includes video games like the Super Mario, Grand Theft Auto for personal computers or video game consoles. These games can be created using CGI (computer generated imagery) that can be designed by applications like Maya, 3ds Max etc.
Also a software usually runs on a software platform which can either be provided by the Operating System or by OS independent platforms like Java and .NET. Software written for one platform is usually unable to run on other platforms so that for instance, Microsoft Windows software will not be able to run on Mac OS because of the differences relating to the platforms and their own standards. These applications can work using software porting, interpreters or re-writing the source code for that platform.
Relationship to computer hardware
Computer software is so called to distinguish it from computer hardware, which encompasses the physical interconnections and devices required to store and execute (or run) the software. At the lowest level, software consists of a machine language specific to an individual processor. A machine language consists of groups of binary values signifying processor instructions which change the state of the computer from its preceding state. Software is an ordered sequence of instructions for changing the state of the computer hardware in a particular sequence. It is usually written in high-level programming languages that are easier and more efficient for humans to use (closer to natural language) than machine language. High-level languages are compiled or interpreted into machine language object code. Software may also be written in an assembly language, essentially, a mnemonic representation of a machine language using a natural language alphabet. Assembly language must be assembled into object code via an assembler.
The term "software" was first used in this sense by John W. Tukey in 1958.[3] In computer science and software engineering, computer software is all computer programs. The theory that is the basis for most modern software was first proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem.
NOTE:
Computer software, or just software is a general term used to describe a collection of computer programs, procedures and documentation that perform some tasks on a computer system.
A screenshot of the OpenOffice.org Writer desktop software
The term includes:Application software such as word processors which perform productive tasks for users, System software such as operating systems, which interface with hardware to provide the necessary services for application software, and Middleware which controls and co-ordinates distributed systems.
Software includes websites, programs, video games etc. that are coded by programming languages like C, C++, etc.
Firmware which is software programmed resident to electrically programmable memory devices on board mainboards or other types of integrated hardware carriers
Testware which is an umbrella term or container term for all utilities and application software that serve in combination for testing a software package but not necessarily may optionally contribute to operational purposes. As such, testware is not a standing configuration but merely a working environment for application software or subsets thereof.
"Software" is sometimes used in a broader context to mean anything which is not hardware but which is used with hardware, such as film, tapes and records.
Overview
Computer software is often regarded as anything but hardware, meaning that the "hard" are the parts that are tangible (able to hold) while the "soft" part is the intangible objects inside the computer. Software encompasses an extremely wide array of products and technologies developed using different techniques like programming languages, scripting languages etc. The types of software include web pages developed by technologies like HTML, PHP, Perl, JSP, ASP.NET, XML, and desktop applications like Microsoft Word, OpenOffice developed by technologies like C, C++, Java, C#, etc. Software usually runs on an underlying operating system (which is a software also) like Microsoft Windows, Linux (running GNOME and KDE), Sun Solaris etc. Software also includes video games like the Super Mario, Grand Theft Auto for personal computers or video game consoles. These games can be created using CGI (computer generated imagery) that can be designed by applications like Maya, 3ds Max etc.
Also a software usually runs on a software platform which can either be provided by the Operating System or by OS independent platforms like Java and .NET. Software written for one platform is usually unable to run on other platforms so that for instance, Microsoft Windows software will not be able to run on Mac OS because of the differences relating to the platforms and their own standards. These applications can work using software porting, interpreters or re-writing the source code for that platform.
Relationship to computer hardware
Computer software is so called to distinguish it from computer hardware, which encompasses the physical interconnections and devices required to store and execute (or run) the software. At the lowest level, software consists of a machine language specific to an individual processor. A machine language consists of groups of binary values signifying processor instructions which change the state of the computer from its preceding state. Software is an ordered sequence of instructions for changing the state of the computer hardware in a particular sequence. It is usually written in high-level programming languages that are easier and more efficient for humans to use (closer to natural language) than machine language. High-level languages are compiled or interpreted into machine language object code. Software may also be written in an assembly language, essentially, a mnemonic representation of a machine language using a natural language alphabet. Assembly language must be assembled into object code via an assembler.
The term "software" was first used in this sense by John W. Tukey in 1958.[3] In computer science and software engineering, computer software is all computer programs. The theory that is the basis for most modern software was first proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem.
NOTE:
Refrigerant Management Software
Machinery Manufacturing Software
Fiscal Declaration Accountancy
Multijunction photovoltaic cell
Multijunction photovoltaic cells are a sub-class of solar cell or photovoltaic cell developed for higher efficiency. These multijunction cells consist of multiple thin films produced using molecular beam epitaxy and / or Metalorganic vapour phase epitaxy. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
History
Multijunction solar cells first were developed and deployed for Satellite power applications where the high cost was offset by the weight savings offered by the higher efficiency.
Multijunction cells have recently seen application in terrestrial applications in Concentrated photovoltaics. The combination of the higher efficiency and concentration has resulted in a price competitive with silicon flat panel arrays.
This technology is currently being utilized in the Mars rover missions.[1]
Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise. In just the past 12 months (12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000-$1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.
Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2005 and 2007.
Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date. The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL[2].
Theory of operation
In a single band gap solar cell, efficiency is limited due to the inability to efficiently convert the broad range of energy that photons possess in the solar spectrum. Photons below the band gap of the cell material are lost; they either pass through the cell or are converted to only heat within the material. Energy in the photons above the band gap energy is also lost, since only the energy necessary to generate the hole-electron pair is utilized, and the remaining energy is converted into heat.
By utilizing multiple junctions with several band gaps, different portions of the solar spectrum may be converted by each junction at a greater efficiency.
Device description
Multijunction photovoltaic cells use many layers of Epitaxy deposited films. By using differing alloys of III-V Semiconductors, the band-gap of each layer may be tuned to absorb a specific band of the solar electromagnetic radiation. The ability to optimize the respective band gaps of the various junctions is hampered by the requirement that each layer must be lattice matched to all other layers. (See Lattice constant).
Each layer is optically in series, with the highest band gap material at the top. The first junction receives all of the spectrum. Photons above the band gap of the first junction are absorbed in the first layer. Photons below the band gap of the first layer pass through to the lower layers to be absorbed there.
All currently commercialized cells utilize tandem electrical connection. This means that they are electrically connected in series and the composite cell has two terminals. A major constraint placed upon tandem cells is that because of the series connection, the current through each junction will be the same. If the maximum power point current of each junction is not the same, then efficiency suffers. Current match of each junction is a very important design consideration for multijunction cells.
Material classification
Multijunction cells may be categorized by the substrate used for cell manufacture. Cells on Germanium and Gallium arsenide have been commercialized. Research into Indium Phosphide based cells for lower band gaps is ongoing.
Gallium arsenide substrate
Twin junction cells with Indium gallium phosphide and gallium arsenide can be made on gallium arsenide wafers. Alloys of In.5Ga.5P through In.53Ga.47P may be used as the high band gap alloy. This alloy range provides for the ability to have band gaps in the range of 1.92eV to 1.87eV. The lower GaAs junction has a band gap of 1.42eV.
The considerable quantity of photons in the solar spectrum with energies below the band gap of GaAs results in a considerable limitation on the achievable efficiency of GaAs substrate cells.
In spacecraft applications, the cells have a poor current match due to a greater photon flux of photons above 1.87eV vs. those between 1.87eV and 1.42eV. This results in too little current in the GaAs junction, and hampers the overall efficiency since the InGaP junction operates below MPP current and the GaAs junction operates above MPP current. To improve current match, the InGaP layer is intentionally thinned to allow additional photons to penetrate to the lower GaAs layer.
In terrestrial concentrating applications, the scatter of blue light by the atmosphere reduces the photon flux above 1.87eV, better balancing the junction currents.
Germanium substrate
Triple junction cells consisting of Indium gallium phosphide, Gallium arsenide or Indium gallium arsenide and Germanium can be fabricated on germanium wafers. Early cells used straight gallium arsenide in the middle junction. Later cells have utilized In.015Ga.985As, due to the better lattice match to Ge, resulting in a lower defect density.
Due to the huge band gap difference between GaAs (1.42eV), and Ge (.66eV), the current match is very poor, with the Ge junction operated significantly current limited.
Current efficiencies for InGaP/GaAs/Ge cells are in the mid 30% range.
Research into methods to produce bang gaps in the range between the Ge and GaAs is ongoing. Lab cells using additional junctions between the GaAs and Ge junction have demonstrated efficiencies above 40%.
Indium phosphide substrate
Indium Phosphide may be used as a substrate to fabricate cells with band gaps between 1.35eV and 0.74eV. Indium Phosphide has a band gap of 1.35eV. Indium gallium arsenide (In0.53Ga.47As) is lattice matched to Indium Phosphide with a band gap of 0.74eV. A quaternary alloy of Indium gallium arsenide phosphide can be lattice matched for any band gap in between the two.
Indium Phosphide based cells are being researched as a possible companion to gallium arsenide cells. The two differing cells may be either optically connected in series (with the InP cell below the GaAs cell), or through the use of spectra splitting using a Dichroic filter.
NOTE:
History
Multijunction solar cells first were developed and deployed for Satellite power applications where the high cost was offset by the weight savings offered by the higher efficiency.
Multijunction cells have recently seen application in terrestrial applications in Concentrated photovoltaics. The combination of the higher efficiency and concentration has resulted in a price competitive with silicon flat panel arrays.
This technology is currently being utilized in the Mars rover missions.[1]
Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise. In just the past 12 months (12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000-$1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.
Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2005 and 2007.
Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date. The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL[2].
Theory of operation
In a single band gap solar cell, efficiency is limited due to the inability to efficiently convert the broad range of energy that photons possess in the solar spectrum. Photons below the band gap of the cell material are lost; they either pass through the cell or are converted to only heat within the material. Energy in the photons above the band gap energy is also lost, since only the energy necessary to generate the hole-electron pair is utilized, and the remaining energy is converted into heat.
By utilizing multiple junctions with several band gaps, different portions of the solar spectrum may be converted by each junction at a greater efficiency.
Device description
Multijunction photovoltaic cells use many layers of Epitaxy deposited films. By using differing alloys of III-V Semiconductors, the band-gap of each layer may be tuned to absorb a specific band of the solar electromagnetic radiation. The ability to optimize the respective band gaps of the various junctions is hampered by the requirement that each layer must be lattice matched to all other layers. (See Lattice constant).
Each layer is optically in series, with the highest band gap material at the top. The first junction receives all of the spectrum. Photons above the band gap of the first junction are absorbed in the first layer. Photons below the band gap of the first layer pass through to the lower layers to be absorbed there.
All currently commercialized cells utilize tandem electrical connection. This means that they are electrically connected in series and the composite cell has two terminals. A major constraint placed upon tandem cells is that because of the series connection, the current through each junction will be the same. If the maximum power point current of each junction is not the same, then efficiency suffers. Current match of each junction is a very important design consideration for multijunction cells.
Material classification
Multijunction cells may be categorized by the substrate used for cell manufacture. Cells on Germanium and Gallium arsenide have been commercialized. Research into Indium Phosphide based cells for lower band gaps is ongoing.
Gallium arsenide substrate
Twin junction cells with Indium gallium phosphide and gallium arsenide can be made on gallium arsenide wafers. Alloys of In.5Ga.5P through In.53Ga.47P may be used as the high band gap alloy. This alloy range provides for the ability to have band gaps in the range of 1.92eV to 1.87eV. The lower GaAs junction has a band gap of 1.42eV.
The considerable quantity of photons in the solar spectrum with energies below the band gap of GaAs results in a considerable limitation on the achievable efficiency of GaAs substrate cells.
In spacecraft applications, the cells have a poor current match due to a greater photon flux of photons above 1.87eV vs. those between 1.87eV and 1.42eV. This results in too little current in the GaAs junction, and hampers the overall efficiency since the InGaP junction operates below MPP current and the GaAs junction operates above MPP current. To improve current match, the InGaP layer is intentionally thinned to allow additional photons to penetrate to the lower GaAs layer.
In terrestrial concentrating applications, the scatter of blue light by the atmosphere reduces the photon flux above 1.87eV, better balancing the junction currents.
Germanium substrate
Triple junction cells consisting of Indium gallium phosphide, Gallium arsenide or Indium gallium arsenide and Germanium can be fabricated on germanium wafers. Early cells used straight gallium arsenide in the middle junction. Later cells have utilized In.015Ga.985As, due to the better lattice match to Ge, resulting in a lower defect density.
Due to the huge band gap difference between GaAs (1.42eV), and Ge (.66eV), the current match is very poor, with the Ge junction operated significantly current limited.
Current efficiencies for InGaP/GaAs/Ge cells are in the mid 30% range.
Research into methods to produce bang gaps in the range between the Ge and GaAs is ongoing. Lab cells using additional junctions between the GaAs and Ge junction have demonstrated efficiencies above 40%.
Indium phosphide substrate
Indium Phosphide may be used as a substrate to fabricate cells with band gaps between 1.35eV and 0.74eV. Indium Phosphide has a band gap of 1.35eV. Indium gallium arsenide (In0.53Ga.47As) is lattice matched to Indium Phosphide with a band gap of 0.74eV. A quaternary alloy of Indium gallium arsenide phosphide can be lattice matched for any band gap in between the two.
Indium Phosphide based cells are being researched as a possible companion to gallium arsenide cells. The two differing cells may be either optically connected in series (with the InP cell below the GaAs cell), or through the use of spectra splitting using a Dichroic filter.
NOTE:
Legacy Transformation Software
Power semiconductor device
Power semiconductor devices are semiconductor devices used as switches or rectifiers in power electronic circuits (switch mode power supplies for example). They are also called power devices or when used in integrated circuits, called power ICs.
Some common power devices are the power diode, thyristor, power MOSFET and IGBT (insulated gate bipolar transistor). A power diode or MOSFET, for example, operates on similar principles as its low-power counterpart, but is able to carry a larger amount of current and typically is able to support a larger reverse-bias voltage in the off-state.
Structural changes are often made in power devices to accommodate the higher current density, higher power dissipation and/or higher reverse breakdown voltage. The vast majority of the discrete (i.e non integrated) power devices are built using a vertical structure, whereas small-signal devices employ a lateral structure. With the vertical structure, the current rating of the device is proportional to its area, and the voltage blocking capability is achieved in the height of the die. With this structure, one of the connections of the device is located on the bottom of the semiconductor [die].
Power semiconductor devices are only used in commutation mode (i.e they are either on or off), and are therefore optimised for this. Most of them should not be used in linear operation.
History
Power semiconductor devices appeared in 1952 with the introduction of the power diode by R.N. Hall. It was made of Germanium and had a voltage capability of 200 volts and a current rating of 35 amperes. Then appeared the thyristor in 1957. They are able to withstand very high reverse breakdown voltage and are also capable of carrying high current. One disadvantage of the thyristor is that once it is 'latched-on' in the conducting state it cannot be turned off by external control. The thyristor turn-off is passive, i.e., the power must be disconnected from the device. This is a major disadvantage for switching circuits.
Although bipolar transistors where invented in 1948, the first devices with substantial power handling capabilities where introduced in the 1960s. These components overcome the limitations of the thyristors, as they can be turned on or off.
With the improvements of the Metal Oxide Semiconductor technology (initially developed to produce integrated circuits), power MOSFETs became available in the late 1970s. International Rectifier introduced a 25 A, 400 V power MOSFET in 1978.[1] These devices allow operation at higher frequency than bipolar transistors, but are limited to the low voltage applications.
Developed in the 1980s, the Insulated Gate Bipolar Transistor (IGBT) became widely available in the 1990s. This component has the power handling capability of the bipolar transistor, with the advantages of the isolated gate drive of the power MOSFET. It has since almost completely replaced the bipolar transistor in power applications.
Common power semiconductor devices
The realm of power devices is divided into two main categories (see figure 1):The two-terminal devices (diodes), whose state is completely dependent on the external power circuit they are connected to; The three-terminal devices, whose state is not only dependent on their external power circuit, but also on the signal on their driving terminal (gate or base). Transistors and thyristors belong to that category.
A second classification is less obvious, but has a strong influence on device performance: Some devices are majority carrier devices (Schottky diode, MOSFET), while the others are minority carrier devices (Thyristor, bipolar transistor, IGBT). The former use only one type of charge carriers, while the latter use both (i.e electrons and holes). The majority carrier devices are faster, but the charge injection of minority carrier devices allows for better On-state performance.
Diodes
An ideal diode should have the following behaviour:When forward-biased, the voltage across the end terminals of the diode should be zero, whatever the current that flows through it (on-state); When reverse-biased, the leakage current should be zero whatever the voltage (off-state).
Moreover, the transition between on and off states should be instantaneous.
In reality, the design of a diode is a trade-off between performance in on-state, off-state and commutation. Indeed, it is the same area (actually the lightly-doped region of a PiN diode) of the device that has to sustain the blocking voltage in off-state and allow current flow in the on-state. As the requirements for the two state are completely opposite, it can be intuitively seen that a diode has to be either optimised for one of them, or time must be allowed to switch from one state to the other (i.e slow down the commutation speed).
This trade-off between on-state, off-state and switching speed is the same for all power devices. A Schottky diode has excellent switching speed and on-state performance, but a high level of leakage current in off-state. PiN diodes are commercially available in different commutation speeds (so-called fast rectifier, ultrafast rectifier...), but any increase in speed is paid by lower performance in on-state.
Switches
The trade-off between voltage, current and frequency ratings also exists for the switches. Actually, all power semiconductors rely on a PiN diode structure to sustain voltage. This can be seen in figure 2. The power MOSFET has the advantages of the majority carrier devices, so it can achieve very high operating frequency, but can't be used with high voltages. As it is a physical limit, no improvement is expected from silicon MOSFET concerning their maximum voltage ratings. However, its excellent performance in low voltage make it the device of choice (actually the only choice) for applications below 200 V. By paralleling several devices, it is possible increase the current rating of a switch. The MOSFET is particularly suited to this configuration because its positive thermal coefficient of resistance tends to balance current between individual devices.
The IGBT is a relatively new component, so its performance improves regularly as technology evolves. It has already completely replaced the bipolar transistor in power applications, and the availability of power modules (in which several IGBT dice are connected in parallel) makes it attractive for power levels up to several megawatts, pushing further the limit where thyristors and GTO become the only option. Basically, an IGBT is a bipolar transistor driven by a power MOSFET: it has the advantages of being a minority carrier device (good performance in on-state, even for high voltage devices), with the high input impedance of a MOSFET (it can be driven on or off with a very low amount of power). Its major limitation for low voltage applications is the relatively high voltage drop it exhibits in on-state (2 to 4 V). Compared to the MOSFET, the operating frequency of the IGBT is relatively low (few devices are rated over 50 kHz), mainly because of a so-called 'current-tail' problem during turn-off. This problem is caused by the slow decay of the conduction current during turn-off resulting from slow recombination of large number of carriers, which flood the thick 'drift' region of the IGBT during conduction. The net result is that the turn-off switching loss of an IGBT is considerably higher than its turn-off loss. Generally, in datasheet, turn-off energy is mentioned as a measured parameter and one has to multiply that number with the switching frequency of the intended application to estimate the turn-off loss.
At very high power levels, thyristor-based devices (SCR, GTO, MCT) are still the only choice. Though driving a thyristor is somewhat complicated, as this device can only be turned on. It turns off by itself as soon as no more current flows through it. This requires specific circuit with means to divert current, or specific applications where current is known to cancel regularly (i.e Alternating Current). Different solution have been developed to overcome this limitation (Mos Controlled Thyristors, Gate Turn Off thristor...). These components are widely used in power distribution applications.
NOTE:
Some common power devices are the power diode, thyristor, power MOSFET and IGBT (insulated gate bipolar transistor). A power diode or MOSFET, for example, operates on similar principles as its low-power counterpart, but is able to carry a larger amount of current and typically is able to support a larger reverse-bias voltage in the off-state.
Structural changes are often made in power devices to accommodate the higher current density, higher power dissipation and/or higher reverse breakdown voltage. The vast majority of the discrete (i.e non integrated) power devices are built using a vertical structure, whereas small-signal devices employ a lateral structure. With the vertical structure, the current rating of the device is proportional to its area, and the voltage blocking capability is achieved in the height of the die. With this structure, one of the connections of the device is located on the bottom of the semiconductor [die].
Power semiconductor devices are only used in commutation mode (i.e they are either on or off), and are therefore optimised for this. Most of them should not be used in linear operation.
History
Power semiconductor devices appeared in 1952 with the introduction of the power diode by R.N. Hall. It was made of Germanium and had a voltage capability of 200 volts and a current rating of 35 amperes. Then appeared the thyristor in 1957. They are able to withstand very high reverse breakdown voltage and are also capable of carrying high current. One disadvantage of the thyristor is that once it is 'latched-on' in the conducting state it cannot be turned off by external control. The thyristor turn-off is passive, i.e., the power must be disconnected from the device. This is a major disadvantage for switching circuits.
Although bipolar transistors where invented in 1948, the first devices with substantial power handling capabilities where introduced in the 1960s. These components overcome the limitations of the thyristors, as they can be turned on or off.
With the improvements of the Metal Oxide Semiconductor technology (initially developed to produce integrated circuits), power MOSFETs became available in the late 1970s. International Rectifier introduced a 25 A, 400 V power MOSFET in 1978.[1] These devices allow operation at higher frequency than bipolar transistors, but are limited to the low voltage applications.
Developed in the 1980s, the Insulated Gate Bipolar Transistor (IGBT) became widely available in the 1990s. This component has the power handling capability of the bipolar transistor, with the advantages of the isolated gate drive of the power MOSFET. It has since almost completely replaced the bipolar transistor in power applications.
Common power semiconductor devices
The realm of power devices is divided into two main categories (see figure 1):The two-terminal devices (diodes), whose state is completely dependent on the external power circuit they are connected to; The three-terminal devices, whose state is not only dependent on their external power circuit, but also on the signal on their driving terminal (gate or base). Transistors and thyristors belong to that category.
A second classification is less obvious, but has a strong influence on device performance: Some devices are majority carrier devices (Schottky diode, MOSFET), while the others are minority carrier devices (Thyristor, bipolar transistor, IGBT). The former use only one type of charge carriers, while the latter use both (i.e electrons and holes). The majority carrier devices are faster, but the charge injection of minority carrier devices allows for better On-state performance.
Diodes
An ideal diode should have the following behaviour:When forward-biased, the voltage across the end terminals of the diode should be zero, whatever the current that flows through it (on-state); When reverse-biased, the leakage current should be zero whatever the voltage (off-state).
Moreover, the transition between on and off states should be instantaneous.
In reality, the design of a diode is a trade-off between performance in on-state, off-state and commutation. Indeed, it is the same area (actually the lightly-doped region of a PiN diode) of the device that has to sustain the blocking voltage in off-state and allow current flow in the on-state. As the requirements for the two state are completely opposite, it can be intuitively seen that a diode has to be either optimised for one of them, or time must be allowed to switch from one state to the other (i.e slow down the commutation speed).
This trade-off between on-state, off-state and switching speed is the same for all power devices. A Schottky diode has excellent switching speed and on-state performance, but a high level of leakage current in off-state. PiN diodes are commercially available in different commutation speeds (so-called fast rectifier, ultrafast rectifier...), but any increase in speed is paid by lower performance in on-state.
Switches
The trade-off between voltage, current and frequency ratings also exists for the switches. Actually, all power semiconductors rely on a PiN diode structure to sustain voltage. This can be seen in figure 2. The power MOSFET has the advantages of the majority carrier devices, so it can achieve very high operating frequency, but can't be used with high voltages. As it is a physical limit, no improvement is expected from silicon MOSFET concerning their maximum voltage ratings. However, its excellent performance in low voltage make it the device of choice (actually the only choice) for applications below 200 V. By paralleling several devices, it is possible increase the current rating of a switch. The MOSFET is particularly suited to this configuration because its positive thermal coefficient of resistance tends to balance current between individual devices.
The IGBT is a relatively new component, so its performance improves regularly as technology evolves. It has already completely replaced the bipolar transistor in power applications, and the availability of power modules (in which several IGBT dice are connected in parallel) makes it attractive for power levels up to several megawatts, pushing further the limit where thyristors and GTO become the only option. Basically, an IGBT is a bipolar transistor driven by a power MOSFET: it has the advantages of being a minority carrier device (good performance in on-state, even for high voltage devices), with the high input impedance of a MOSFET (it can be driven on or off with a very low amount of power). Its major limitation for low voltage applications is the relatively high voltage drop it exhibits in on-state (2 to 4 V). Compared to the MOSFET, the operating frequency of the IGBT is relatively low (few devices are rated over 50 kHz), mainly because of a so-called 'current-tail' problem during turn-off. This problem is caused by the slow decay of the conduction current during turn-off resulting from slow recombination of large number of carriers, which flood the thick 'drift' region of the IGBT during conduction. The net result is that the turn-off switching loss of an IGBT is considerably higher than its turn-off loss. Generally, in datasheet, turn-off energy is mentioned as a measured parameter and one has to multiply that number with the switching frequency of the intended application to estimate the turn-off loss.
At very high power levels, thyristor-based devices (SCR, GTO, MCT) are still the only choice. Though driving a thyristor is somewhat complicated, as this device can only be turned on. It turns off by itself as soon as no more current flows through it. This requires specific circuit with means to divert current, or specific applications where current is known to cancel regularly (i.e Alternating Current). Different solution have been developed to overcome this limitation (Mos Controlled Thyristors, Gate Turn Off thristor...). These components are widely used in power distribution applications.
NOTE:
electric loudspeaker processor
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