7.1 INTRODUCTION

The field-effect transistor, customarily known as FET, like bipolar junction transistor (BJT), is a three-terminal semiconductor device extensively used in numerous electronic circuits and systems ranging from digital logic circuits to amplifier design. The term ‘field effect’ arises due to the fact that applied external electric field creates a conduction path called channel and controls the output current of the device. Even though FET has various structural and operational similarities with BJT, they have some fundamental differences, which are as follows:

• In BJT, the current is contributed by two types of carriers: electrons and holes (hence the term ‘bi’, meaning two, which justifies its name bipolar). In case of FET, based on its structure, only one type of carrier, i.e. either electron or hole, contributes to the current. Hence, it is known as a unipolar device. Due to this, FETs do not suffer from minority carrier storage effects and consequently have higher switching speeds and higher cut-off frequencies.

• BJT is a current-control device, where output current IC is solely controlled by the input current IB with a relation IC ≈ βIB. On the other hand, FET is a voltage-controlled device, where the voltage between two terminals VGS controls the output current ID. This is indicated in the schematic block diagram of Figure 7.1. Thus, FET is termed as a voltage-controlled device.

One of the most important features of FET is its very high input impedance, typically much higher than that of BJT. This makes FETs a very good choice for designing linear amplifiers. Another advantageous feature of FETs is their better temperature stability compared to BJT. At high current level, FET offers negative temperature coefficient of current (meaning that current decreases due to increase in temperature). Due to this phenomenon, FETs can prevent thermal runaway. In addition, FETs are much compact in size. All these features make FET a very good candidate for integrated-circuit (IC) chips.

FETs can be broadly classified into two types: a) Junction Field-Effect Transistor (JFET) and b) Metal Oxide Semiconductor Field-Effect Transistor (MOSFET). The latter can be further divided into depletion and enhancement types. JFET and both types of MOSFETs, based on the structures, can be further divided into n-channel and p-channel types. Figure 7.2 shows various types of MOSFET.