Important features of FET

Introduction

• The Junction Field Effect Transistor abbreviated as JFET or simply FET is an another semiconductor device like a BJT which can be used as an amplifier or switch. Like BJT, FET is also a three terminal device; however, the principle of operation of FET is completely different from that of BJT.

• The three terminals of FET are named as Drain (D), Source (S) and Gate (G), as shown in the Fig. 3.1.1. Out of these three terminals gate terminal acts as a controlling terminal.

Important features of FET

• Voltage Controlled Device : As shown in the Fig. 3.1.1, in BJT the output current, Ic is controlled by the base current IR. Hence BJT is a current controlled device. On the other hand, in FET, the voltage applied between gate and source (VGS) controls the drain current ID. Therefore, FET is a voltage controlled device.

• The name field effect is derived from the fact that the output current flow is controlled by an electric field set up in the device by an externally applied voltage between gate and source terminals.

• Unipolar Device : We know that in BJT, the current is carried by both electrons and holes, and hence the name "bipolar" junction transistor. However in FET, current is carried by only one type of charge particles, either electrons or holes. Hence FET is called unipolar device.

• Like BJT, the parameters of FET are also temperature dependent. In FET, as temperature increases drain resistance also increases, reducing the drain current. Thus unlike BJT, thermal runaway does not occur with FET. Thus we can say that FET is more temperature stable as compared to the BJT.

• FET has very high input impedance. Typically, it is in the range of one to several megaohms. Because FETs have higher input impedance than BJT they are preferred in amplifiers where high input impedance is required.

• FETs require less space than that for BJTs, hence they are preferred in integrated circuits.

Review Questions

1. What is FET ?

2. State the important features of FET.