POINT-CONTACT TRANSISTORS

This transistor consists of two electrodes (emitter and collector) which make contact with a germanium, and a third electrode (the base) which is soldered to that pellet. The entire assembly is encased in a plastic housing to avoid the conta­minating effects of the atmosphere.

Germanium crystal pellet.

Cat whiskers

Emitter Collector

Impregnated rugged case

Welded connections

Surface-bound electrons c.

Equivalent potential hill battery

Base

Donor atoms - Electrons

e –Emitter

с – Collector

b – Base

Fig.12. Construction of Point-Contact Transistor (a). Baste Point-Contact Transistor Operation (b).

The pellet is usually N-type germanium. The emitter and collector contacts are metallic wires. Transistor operation re­quires an intense electric field. The high voltage, in the input or emitter circuit, produces a high current which burns out the transistor. In the output or collector circuit, the high vol­tage causes a break-down. Thus, since the battery voltage is limited the use of the point-contact cat whiskers is a conveni­ent method of obtaining the required high-intensity field.

The fundamental concepts of current flow in the point-con­tact transistor are illustrated in Fig. 12. Physicists have found that those electrons which diffuse to the surface of the germa­nium pellet not only lose their ability to return to the interior of the germanium but also form a skin-like covering over the surface.