Metal rectifier

A metal rectifier is an early type of semiconductor rectifier in which the semiconductor is copper oxide or selenium. They were used in power applications to convert alternating current to direct current in devices such as radios and battery chargers. 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.

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).

The principle of operation of a metal rectifier is related to modern semiconductor rectifiers (Schottky diodes and p–n diodes), but somewhat more complex. Both selenium and copper oxide are semiconductors, in practice doped by impurities during manufacture. When they are deposited on metals, it would be expected that the result is a simple metal–semiconductor junction and that the rectification would be a result of a Schottky barrier. However, this is not always the case: the scientist S. Poganski discovered in the 1940s that the best selenium rectifiers were in fact semiconductor-semiconductor junctions between selenium and a thin cadmium selenide layer, generated out of the cadmium-tin metal coating during processing. In any case the result is that there is a depletion region in the semiconductor, with a built-in electric field, and this provides the rectifying action.

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