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Wednesday, 16 October 2019

Metal Semiconductor Rectifiers

Copper Oxide Rectifiers and other metal semiconductor rectifiers are explained below.

Copper Oxide Rectifiers :

It is a plate of 99.98% pure copper on which a film of cuprous oxides is produced by a special process at high temperature. One side of the plate is cleaned of cuprous oxide and an electrode is soldered directly to copper. The second electrode is soldered to cuprous oxide film. When positive potential is applied to the oxide layer and negative to the copper, it corresponds to forward biasing a P-N junction. The rectifiers, for many kinds of measuring circuits and instruments, can be obtained by arranging the copper plate elements in stacks. Figure shows the arrangements in the copper oxide rectifier.

These rectifiers have low permissible current density and they are not used for power supply purposes. These rectifiers are protected against moisture by giving their elements a coating of insulating varnish and are often sealed in hermetic containers for protection against moisture. These rectifiers completely fail at sufficiently high reverse voltage. These are cheaper than silicon rectifiers and have better frequency response and do not have distortion in the rectified wave form.

Selenium Rectifier :

These rectifiers use 99.9% pure selenium. Purity is very important in respect of permissible current density and wider working temperature range as compared to copper oxide rectifier. They are used in battery charging and electro plating supplies. The typical selenium rectifier are shown in Figure.

Germanium Rectifiers :

The thermionic valves have been replaced by P-N junction diodes. Be and Si are used in P-N junction rectifiers. It is easier and simpler to produce germanium monocrystals. Germanium melts at 958°C and therefore easier to purify and maintain free from impurity. Germanium rectifiers have limited working temperature from - 50 to + 70°C. Continuous operation at 60°C may cause thermal ageing and deterioration of electrical properties. At low temperature there is a considerable drop in the permissible reverse voltages. It can operate at high current densities and reverse voltages with about 98% efficiency.

Silicon Rectifiers :

Silicon rectifiers can operate upto 200°C. Its melting point is 1415°C. Modern silicon combines easily with practically all chemical elements and is therefore very difficult to purify and maintain free from impurity. For heavy duty application this type of rectifiers are cost suited. Silicon rectifiers are more sensitive to weak signals when used in high frequency electronic circuits.

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