Field-effect transistors

Abstract

Introduction In this chapter we introduce the second major type of transistor: the field-effect transistor . Like the bipolar junction transistors (BJTs) we studied in Chapter 4, field-effect transistors (FETs) allow the user to control a current with another signal. The key difference is that the FET control signal is a voltage while the BJT control signal is a current. Also, the FET control input (called the gate) has a much higher input impedance than the base of a BJT. Indeed, the DC gate impedance for FETs varies from a few megaohms to astounding values in excess of 10 14 Ω. High input impedance is a highly desirable feature that greatly simplifies circuit analysis. The BJT has three connections: the collector, base, and emitter. The corresponding connections on an FET are called the drain, gate , and source . Some versions of the FET have a fourth connection called the bulk connection. Bipolar transistors come in just two types with opposite polarities: the npn and the pnp. Field-effect transistors have greater variety. In addition to the polarity pairs (termed n-channel and p-channel ), there are differences in gate construction ( junction and metal oxide ), and doping ( depletion and enhancement ). In terms of analysis, however, they are all very similar, so we will not have to consider each variety separately. Also, as we did with the bipolar transistor, we will focus on one of the polarities (n-channel) since the other polarity simply involves swapping the labels for n and p and changing the sign of the voltages and the direction of the currents.