2.1 Introduction

The integrated circuit made modern day information processing and communications systems possible. Its basic functional element is the transistor, most commonly a silicon metal oxide semiconductor field-effect transistor (MOSFET). For the past forty years, MOSFET scaling (the reduction of its critical dimension by a factor of about each technology generation, approximately 18 months) has driven Moore's Law (the doubling of the number of transistors per integrated circuit each technology generation). It now appears that the silicon MOSFET will reach its scaling limit within a decade or so [2.1, 2.2], and devices to complement or replace the silicon MOSFET are being explored [2.3].

This chapter is a brief overview of MOSFET essentials and a quick introduction to the bipolar transistor. The chapter provides some context for exploring new devices and an opportunity to discuss three important points. First, we discuss charge control by a gate electrode, which modulates the transistor's current. Electrostatics is likely to be similarly important for transistors that follow the MOSFET. Second, we discuss the characteristics of devices that make them useful in high-density, high-speed digital systems. Finally, we examine the fundamental limits that apply to any electronic switching device used for conventional, digital logic.

2.2 The MOSFET

Figure 2.1 illustrates the physical structure of two different kinds of MOSFETs. An n-channel bulk MOSFET is built on a p-type substrate with deep n ⁺ regions to facilitate contact to the source and drain. Shallow n ⁺ junctions connect the source and drain to...