Wide-Spaced Transistors

Author: Leonard Krugman

It was noted previously that the emitter and collector contacts of the point-contact transistor must be closely spaced for normal transistor action, since the functioning of this transistor requires an intense electric field. In addition, the frequency response of this transistor decreases rapidly with increased contact spacing. Theoretically, the frequency operating band varies inversely as the cube of the contact spacing. In spite of this, it has been found that wide-spaced transistors have some novel and useful characteristics. When germanium having lower conductivity (fewer impurity atoms) is used, an increase in the normal contact spacing from .002 inch to as much as .015 inch has no effect on the transistor current and power gains. At the same time, the effect of the emitter voltage on the collector current is decreased, due to increased spacing.

The ratio of change in emitter voltage to the resulting change in collector current defines the backward transfer or feedback resistance. The feedback resistance in a transistor acts similarly to the positive feedback parameter in a vacuum tube circuit. (The feedback resistance and other related transistor characteristics are considered in greater detail in Chapter 3.) The feedback resistance of a transistor with a normal .002 inch contact spacing is about 200 ohms. This resistance is reduced to approximately 50 ohms when the contact spacing is increased to .015 inch. This low value insures circuit stability at relatively high values of power gain.

Germanium with higher than normal resistivity is used to compensate for the narrowing of the usable frequency limits by the wide contact spacing. Despite this, the usable frequency range is reduced to about 1/50 of its normal value. The increased contact spacing has little effect upon other transistor characteristics.