5.1 Introduction

In a conventional MOSFET, carriers are confined in a direction normal to the channel, and free to move in two dimensions. In the previous two chapters, we discussed transistors based on such 2D carriers. It is, however, now possible to make structures that confine carriers in two dimensions, so that they are free to move only in one direction. We refer to such structures as 1D nanowires, and during the past few years, the ability to fabricate field-effect transistors from nanowires has progressed rapidly [5.1 5.5]. Our purpose in this chapter is to discuss the theory of nanowire field-effect transistors using two examples - the semiconductor (specifically, silicon) nanowire FET and the carbon nanotube FET. We begin with a discussion of a simple theory for the ballistic silicon nanowire FET.