5 - Transistors II: FETs

Summary

In this chapter we introduce the field-effect transistor (FET). A majority of today's integrated circuits are built using FETs of one type or another. FET operation is easier to explain than that of bipolar transistors; however, due to the variability of FET parameters, many people find FETs more difficult to use. As with bipolar technologies, it is essential that you master the basics of FET operation, and you will find that knowledge useful later on.

Apparatus required

Breadboard, oscilloscope, multimeter, two 2N5485 JFETs, one 1N4733 Zener diode, two 1 k, one 3.3 k, two 10 k, one 100 k, and one 1 M 14 Wresistors, 0.1μF ceramic capacitor, 1.0μF and 100μF electrolytic capacitors.

Field-effect transistors

Like bipolar junction transistors, field-effect transistors (FETs) are three terminal semiconductor devices capable of power gain. Qualitatively, they operate much like junction transistors, but they have much higher input impedance and lower transconductance and voltage gain. Also, they have a larger variation in their ‘V_BE’ equivalent (called V_GS) than bipolar transistors. They come in a confusing variety of types, but we will concentrate for today on junction FETs (JFETs).

Fundamentally, there are two types of FETs: junction FETs and metaloxide- semiconductor FETs (MOSFETs). In both types, a conducting channel between the drain and source terminals is controlled by a voltage applied to the gate terminal. The channel can be made of either N-type or P-type material (Fig. 5.1). N-channel is more common since the conductivity of N-type semiconductor (in which electrons carry the current) is higher than that of P-type (in which holes do).