Metal rectifier is an early type of semiconductor device in which the semiconductor is copper oxide or selenium. Westinghouse Electric (1886) was a major manufacturer of these rectifiers, under the trade name Westector (now used as a trade name for an overcurrent trip device by Westinghouse Nuclear).
In some countries the term "Metal Rectifier" is applied to all such devices; in others the term "Metal Rectifier" normally refers to copper-oxide types, and "Selenium Rectifier" to selenium-iron types.
Description
Metal rectifiers consist of washer-like discs of different metals, either copper (with an oxide layer to provide the rectification) or steel plated with selenium, interspersed with aluminium discs (which were often of a larger size, to provide cooling).
Performance
Compared to later silicon or germanium devices, copper-oxide rectifiers tended to have poor efficiency, and the reverse voltage rating was rarely more than a few volts. A number of rectifier discs would need to be used in series to provide an adequate reverse breakdown voltage figure - a bridge rectifier for a 12V battery charger would often use 12 metal rectifiers. Selenium rectifiers were generally more efficient than metal-oxide types, and could handle higher voltages. However, considerably more skill was required for their construction.
Applications
Metal rectifiers were also used as envelope detector (AM demodulator) diodes in radio receivers. The WX6 Westector was a typical example. It was about the size and shape of an AAA battery, with threaded posts at each end to which connections were made.
Selenium rectifiers were once widely used as high-tension rectifiers in transformerless radio and TV sets, before cheaper silicon diodes became available. Although they were reasonably efficient in this application, (at least compared to vacuum-tube rectifiers), their internal resistance tended to increase as they aged. Apart from reducing the available high voltage, this tends to make them run hotter, producing an unpleasant smell as the selenium starts to evaporate.
Specially designed selenium rectifiers were once widely used as EHT rectifiers in television sets and photocopiers. A layer of selenium was applied to a sheet of soft iron foil, and thousands of tiny discs (typically 2mm diameter) were punched out of this and assembled as "stacks" inside ceramic tubes. Rectifiers capable of supplying tens of thousands of volts could be made this way. Their internal resistance was extremely high, but most EHT applications only required a few hundred microamps at most, so this was not normally an issue. With the development of inexpensive high voltage silicon rectifiers, this technology has fallen into disuse.
Metal rectifiers have been replaced by silicon diodes in most devices, however there are certain applications where the replacement of metal rectifiers with silicon units has proven impractical. These are mostly in electroplating, aluminium smelting and similar high-current low-voltage industrial applications, where the lower forward voltage drop of metal rectifiers is more important than their reverse breakdown voltage.
Many purpose-designed transformer-rectifier ("Rectiformer") units were specifically designed around the characteristics of metal rectifiers, and substituting silicon rectifiers would necessitate replacing the complete assembly. As is the case with many other industrial processes, it is often considerably cheaper and more convenient to pay someone to manufacture replacement "legacy" metal rectifiers than to upgrade the equipment. Hence, metal rectifiers are still manufactured in small quantities today.
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2008年11月7日星期五
Heterostructure barrier varactor diode
The Heterostructure barrier varactor (HBV) diode was invented by Erik Kollberg together with Anders Rydberg in 1989 at Chalmers University of Technology. This semiconductor diode has an anti-symmetric current-voltage relationship and a symmetric capacitance-voltage relationship as shown in the graph below. Also, in this figure the inset shows the circuit schematic symbol of the HBV. We can see from this symbol that, in practice, the HBV consists of two, back to back, anti-serially connected rectifying diodes (such as Schottky diodes for instance). The gap in the middle of the diode symbol represents the inherent capacitance of the device. The main use of the HBV is as a signal source in the mm-wave and THz frequency spectra. The electrical characteristics of the HBV are realized by separating two layers of a semiconductor material (A) with a layer of another semiconductor material (B). The band-gap of material (B) should be larger than for material (A). This results in a barrier for the carriers trying to travel trough the layers (A)-(B)-(A). The (A) layers are usually n-doped which means that electrons are the majority carriers of this device. At different bias voltages the carriers are redistributed and the distance between the carriers on each side of the barrier (B) is different. As a consequence the HBV has electrical properties resembling the parallel plate capacitor with a voltage dependent plate distance d.
The main application for the HBV diode is to generate signals at extremely high frequencies (> 100 GHz) by multiplying lower frequency signals. Since it is considerably easier to generate AC power at lower frequencies. This lower frequency power can then be multiplied to higher frequencies with the HBV. This is made possible by the characteristic voltage dependence of the capacitance C(V), making the HBV diode highly nonlinear. This property is used to generate higher harmonics f3, f5, f7… from an initial input signal f1 . In other words f3 =3x f1 and f5 =5x f1 etc. So, in effect, the HBV is able to multiply an incoming signal to higher, odd frequency multiples 3,5,7… The reason why the even multiplications, 2,4,6…, are cancelled is because of the symmetric/anti-symmetric nature of the electrical properties. Also, using this inherent symmetry of the device we can operate it without any DC-biasing. This is an advantage compared to the Schottky diode which has to be biased.
Signals at these frequencies (100 GHz - 3 THz) have applications in diverse areas such as radioastronomy, security imaging, biological and medical imaging and high-speed wireless communications.
NOTE:
The main application for the HBV diode is to generate signals at extremely high frequencies (> 100 GHz) by multiplying lower frequency signals. Since it is considerably easier to generate AC power at lower frequencies. This lower frequency power can then be multiplied to higher frequencies with the HBV. This is made possible by the characteristic voltage dependence of the capacitance C(V), making the HBV diode highly nonlinear. This property is used to generate higher harmonics f3, f5, f7… from an initial input signal f1 . In other words f3 =3x f1 and f5 =5x f1 etc. So, in effect, the HBV is able to multiply an incoming signal to higher, odd frequency multiples 3,5,7… The reason why the even multiplications, 2,4,6…, are cancelled is because of the symmetric/anti-symmetric nature of the electrical properties. Also, using this inherent symmetry of the device we can operate it without any DC-biasing. This is an advantage compared to the Schottky diode which has to be biased.
Signals at these frequencies (100 GHz - 3 THz) have applications in diverse areas such as radioastronomy, security imaging, biological and medical imaging and high-speed wireless communications.
NOTE:
industrial mechanic components
active semiconductor components
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