Selenium rectifier

Selenium rectifier Selenium rectifier, also called selenium cell, is a selenium-based semiconductor diode. Let us examine the selenium rectifier structure. The selenium rectifier consists of an aluminum base, one side of which is covered with crystalline selenium that is one of the electrodes with hole conductivity (p-type). To create the other electrode the selenium layer surface is covered with tin, cadmium and bismuth alloy. The reaction of selenium and cadmium result in creation of a thin layer of cadmium selenide with electronic (n-type) conductivity. At the border line between selenium and cadmium selenide a p-n junction appears. To improve the selenium plates properties these undergo electric molding by way of prolonged backward constant current application. Selenium rectifiers bases can have a circular or a rectangular form and a central aperture for assembling in columns. Barrier capacitance of selenium rectifiers is high, which limits their application in high-frequency devices. The parameters of selenium rectifiers change in the course of time. Long-time storage increases backward current. To return the initial value of the backward current, the rectifier must be energized with backward or alternating voltage. Selenium rectifiers are subject to irreversible backward current increase called ageing. In storage the process manifests insignificantly, but accelerates in service. Its intensity increases with temperature growth, which determines the maximum operating temperature. The advantage of such rectifiers consists in their capacity to endure short-term overloads and quickly restore the properties after a breakdown.