Analog MOS Integrated Circuits for Signal Processing

Chapter Three

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 basic properties of MOS capacitors and switches are described, along with their imperfections and limitations. The fabrication process is also briefly described.

The discussions given here are in the simplest possible terms, and are aimed at providing some physical understanding of the highly complex device operation for the circuit designer. Precision and depth were regretfully sacrificed in the process. The ambitious reader is referred to the excellent specialized works listed as references at the end of the chapter.

3.1. SEMICONDUCTORS

In metals (e.g., aluminum, copper, silver, etc.) which 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 (of 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 ...