Field Effect Transistors

Electron Devices

Chapter 5: Field Effect Transistors

Introduction

• The Field Effect Transistor abbreviated as 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 Fig. 5.1.1. Out of these three terminals gate terminal acts as a controlling terminal.

Features of FET

• Voltage Controlled Device: As shown in Fig. 5.1.1, in BJT the output current, I_C is controlled by the base current I_B. Hence BJT is a current controlled device. On the other hand, in FET, the voltage applied between gate and source (V_GS) controls the drain current I_D. 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.

Classification of FET

The FETs are categorised as :

• Junction Field Effect Transistors (JFETs),

• Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

The JFETs are further classified into two types as, n‒channel JFET and p‒channel JFET and MOSFETs are further classified into two types as, depletion MOSFET and enhancement MOSFET. Fig. 5.1.2 shows the different types of FETs.

• In JFET, there is a direct electrical connection. between the gate terminal and the channel of a JFET.

• On the other hand, in MOSFETs, the gate is insulated from the channel by a very thin layer of dielectric material, silicon dioxide (SiO₂). Thus, in MOSFETs, there is no direct electrical connection between the gate terminal and the channel. Due to this extra layer the input resistance of MOSFET is very very high.

Review Questions

1. What is FET?

2. What is JFET ?

3. List important features of FET.

4. Give the classification of FET.