A diode designed as a semiconductor device specifically acts as a one-way switch for current. It permits flow of current easily in one direction, but severely restricts current from flowing in the opposite direction. A metal rectifier is an early version of semiconductor with copper oxide or selenium as a semicinductor material. These diode widely used in power applications to convert alternating current to direct current in devices such as radios and battery chargers. Metal rectifiers made up of washer-like discs of several metals. Which is either copper (with an oxide layer to provide the rectification) or steel or aluminum, plated with selenium. The discs often separated by spacer sleeves to supply cooling for the device.
The principle of operation of metal rectifier somewhat similar to the modern semiconductor rectifiers (Schottky diodes and p–n diodes). But somewhat more complex. Both selenium and copper oxide used as semiconductors, in practice doped by impurities during manufacture. Once they are deposited on metals, it might be expected that the result’s an easy metal-semiconductor junction which the rectification would be a results of a Schottky barrier. generated out of the cadmium-tin metal coating during processing. In any case the result’s that there’s a depletion region within the semiconductor, with a built-in field , and this provides the rectifying action.