Ian Hickman

Overview

Unlike semiconductor diodes, transistors did not see active service in the Second World War; they were born several years too late. In 1948 it was discovered that if a point contact diode detector were equipped with two cat's whiskers rather than the usual one, spaced very close together, the current through one of them could be influenced by a current through the other. The crystal used was germanium, one of the rare earths, and the device had to be prepared by discharging a capacitor through each of the cat's whiskers in turn to "form" a junction. Over the following years, the theory of conduction via junctions was elaborated as the physical processes were unraveled, and the more reproducible junction transistor replaced point contact transistors.

However, the point contact transistor survives to this day in the form of the standard graphical symbol denoting a bipolar junction transistor (Figure 4.1(A)). This has three separate regions, as in Figure 4.1(B), which shows (purely diagrammatically and not to scale) an NPN junction transistor. With the base (another term dating from point contact days) short-circuited to the emitter, no collector current can flow since the collector/ base junction is a reverse biased diode, complete with depletion layer as shown. The higher the reverse bias voltage, the wider the depletion layer, which is found mainly on the collector side of the junction since the collector is more lightly doped than the base. In fact, the pentavalent atoms which make the...