Introduction to CMOS OP-AMPs and Comparators

CHAPTER 2

MOS DEVICES AS CIRCUIT ELEMENTS

In this chapter the physics of MOS (metal-oxide semiconductor) devices is discussed briefly. The most important and simplest current–voltage relations are given, and simple models introduced for MOS transistors in linear operation. The discussion here is in the simplest possible terms, aimed at providing some physical understanding of the highly complex device operation for the circuit designer. Precision and depth have regretfully been sacrificed in the process. The ambitious reader is referred to the excellent specialized works listed as references at the end of the chapter.

2.1. SEMICONDUCTORS

In metals (e.g., aluminum, copper, silver) that are good electrical conductors, the atoms are arranged in a regular crystal array. The electrons from the outer (valence) shell of the atoms are free to move within the material. Since the number of atoms, and thus the number of free electrons, is very large (on the order of 10²³ cm⁻³), even a small electric field results in a large electron current—hence the high conductivity observed for these metals.

The picture is quite different for an insulator such as silicon dioxide (SiO₂). Here the valence electrons form the bonds between adjacent atoms and hence are themselves tied to these atoms. Thus no free electrons are available for conduction and the conductivity is very low.

Semiconductors (such as silicon or germanium) are in between conductors and insulators in their electrical properties. At very ...