The metal-oxide-semiconductor field-effect transistor (MOSFET) is the building block of VLSI circuits in microprocessors and dynamic memories. Because the current in a MOSFET is transported predominantly by carriers of one polarity only (e.g., electrons in an n-channel device), the MOSFET is usually referred to as a unipolar or majority-carrier device. Throughout this chapter, n-channel MOSFETs are used as an example to illustrate device operation and derive drain-current equations. The results can easily be extended to p-channel MOSFETs by exchanging the dopant types and reversing the voltage polarities.

The basic structure of a MOSFET is shown in Fig. 3.1. It is a four-terminal device with the terminals designated as gate (subscript g), source (subscript s), drain (subscript d), and substrate or body (subscript b). An n-channel MOSFET, or nMOSFET, consists of a p-type silicon substrate into which two n ⁺ regions, the source and the drain, are formed (e.g., by ion implantation). The gate electrode is usually made of metal or heavily doped polysilicon and is separated from the substrate by a thin silicon dioxide film, the gate oxide. The gate oxide is usually formed by thermal oxidation of silicon. In VLSI circuits, a MOSFET is surrounded by a thick oxide called the field oxide to isolate it from the adjacent devices. The surface region under the gate oxide between the source and drain is called the channel region and is critical for current conduction in a...