Chapter 4 explores the characteristics of field-effect transistors (FETs). The metal-oxide-semiconductor field-effect transistor (MOSFET) is without doubt the most commercially successful solid-state device. It is the primary component in high-density VLSI chips, including microprocessors and memories. A second type of FET, the junction field-effect transistor (JFET), is based on a pn junction structure and finds application particularly in analog and RF circuit design.

P-channel MOS (PMOS) transistors were the first MOS devices to be successfully fabricated in large-scale integrated (LSI) circuits. Early microprocessor chips used PMOS technology. Greater performance was later obtained with the commercial introduction of n-channel MOS (NMOS) technology, using both enhancement-mode and ion-implanted depletion-mode devices.

This chapter discusses the qualitative and quantitative i-v behavior of MOSFETs and JFETs and investigates the differences between the various types of transistors. Techniques for biasing the transistors in various regions of operation are also presented.

Early integrated circuit chips contained only a few transistors, whereas today, the National Technology Roadmap for Semiconductors (NTRS [2]) projects the existence of chips with 1 billion transistors by the year 2010! This phenomenal increase in transistor density has been the force behind the explosive growth of the electronics industry outlined in Chapter 1 and has been driven by our ability to reduce (scale) the dimensions of the transistor without compromising its operating characteristics.

Although the bipolar junction transistor or BJT was successfully reduced to practice before the FET, the FET is conceptually easier to understand and is by far the most commercially important device. Thus, we consider it first. The BJT is discussed in detail in Chapter 5.