Ian Hickman

Overview

An important milestone in the development of modern active semiconductor devices was the field-effect transistor, or FET for short. These did not become generally available until the 1960s, although they were described in detail and analyzed as early as 1952.

Figure 5.1(A) shows the symbols and Figure 5.1(B) and (C) the construction and operation of the first type introduced, the depletion mode junction FET or JFET. In this device, in contrast to the bipolar transistor, conduction is by means of majority carriers which flow through the channel between the source (analogous to an emitter or cathode) and the drain (collector or anode). The gate is a region of silicon of opposite polarity to the source cum channel cum drain. When the gate is at the same potential as the source and drain, its depletion region is shallow and current carriers (electrons in the case of the N-channel FET shown in Figure 5.1(C)) can flow between the source and the drain. The FET is thus a unipolar device, and minority carriers play no part in its action. As the gate is made progressively more negative, the depletion layer extends across the channel, depleting it of carriers and eventually pinching off the conducting path entirely when V _gs reaches V _p, the pinch-off voltage. Thus for zero (or only very small) voltages of either polarity between the drain and the source, the device can be used as a passive voltage-controlled resistor.