2.1 Introduction

In this chapter we describe a simplified structure of a MOS transistor, and we offer a qualitative, intuitive description of how this device operates. This material will give the reader a bird's-eye view of MOS transistor behavior and will naturally lead to the qualitative models of Chap. 3 or even to other qualitative models that the reader is likely to encounter in the course of circuit design activities.

In many circuits, MOS transistors, when turned on, are operated with rather large currents in the strong inversion region of operation. In fact, this region is the only one to which attention is paid in most circuit design texts. The MOS transistor can, however, also be operated with minute currents, in a region of operation called weak inversion. This region is very important for low-power, battery-operated equipment and for today's low-supply-voltage technologies. Thus we make sure to cover it adequately. The same holds for the moderate inversion region between the weak and strong inversion regions.

2.2 MOS Transistor Structure

A simplified structure of an n-channel MOS transistor is shown in Fig. 2.1 (the names n-channel and MOS will be discussed shortly). The transistor is formed on a silicon body or substrate in which dopant (impurity) atoms have been introduced. ^[*] The dopant atoms used here are acceptors, called so because each accepts one valence electron from a silicon atom, leaving in the latter an electron vacancy, called a hole. The...