Monday, January 15, 2018

Point-Contact Diodes

       Point-contact diodes are the oldest microwave semiconductor devices. They were developed during World War II for use in microwave receivers and are still in widespread use as receiver mixers and detectors. The point-contact diode depend on the pressure of contact between a point and a semiconductor crystal for its operation. Figure a and b, illustrates a point-contact diode. A fine berylium-copper, bronze-phosphor,or tungsten wire,called the CATWHISKER, press against the crystal and forms the other part of the diode. During the manufacture of the point-contact diode, a large current is passed from the catwhisker to the silicon crystal for a short period of time. The result of this large current is the formation of a small region of p-type material around the crystal in the vicinity of the point contact. Thus, a pn-junction is formed which behaves in the same way as a normal pn-junction. The end of the catwhisker is one of the terminals of the diode. It has a lower resistance contact to the external circuit. A flat metal plate on which the crystal is mounted forms the lower contact of the diode with the external circuit. Both contacts with the external circuit are low-resistance contacts. The point-contact diode has an advantage over the pn junction diode because the capacitance between the catwhisker and the crystal is less than the capacitance between the two sides of the junction diode. The major disadvantage of a point-contact diode its very small current rating.

A point-contact diode finds applications in
(1) Microwave Circuits.
(2) AM diode detection in AM receivers.
(3) Microwave receiver mixers and detectors.

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