Construction and Electrode Designations

Author: Leonard Krugman

The elements and basic construction details of the point-contact transistor are shown in Fig. 2-1. This transistor consists of two electrodes (emitter and collector) which which make contact with a germanium pellet, and a third electrode (the base) which is soldered to that pellet. (It is common practice to designate the electrodes by e, c, and b - emitter, collector, and base. The practice will be followed in this book.) The entire assembly is encased in a plastic housing to avoid the contaminating effects of the atmosphere.

Fig. 2-1. Construction of point-contact transistor. Courtesy CBS-Hytron.

The pellet is usually N-type germanium, roughly .05 inch in length and .02 inch thick. The emitter and collector contacts are metallic wires, approximately .005 inch in diameter and spaced about .002 inch apart. These contacts are frequently referred to as "cat whiskers." The bend in the cat whiskers, illustrated in Fig. 2-1, is required to maintain pressure against the germanium pellet surface. The practical man will certainly ask, "Why use cat whiskers which are obviously difficult to manufacture and which produce a mechanically weak contact? Let us eliminate the cat whiskers (he goes on) and use a low-resistance soldered contact similar to that used on the base electrode." An answer to this question necessitates an analysis of the point-contact transistor. Transistor operation requires an intense electric field. If the external battery potential is made high enough to produce the required field intensity, this potential has adverse effects on the transistor. 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 breakdown. Thus, since the battery voltage is limited, as shown by the considerations, the use of the point-contact cat whiskers is a convenient method of obtaining the required high-intensity field. The electrical action of the points in concentrating the battery potential to produce a concentrated electric field is analagous to the increased water pressure which is obtained by decreasing the nozzle area of a garden hose.