1.1 MOS Transistors and Switches

Silicon is predominantly used in the fabrication of semiconductor devices and microcircuits. A MOS (Metal-Oxide-Silicon) transistor structure is built by stacking several layers of conducting, insulating and semiconductor materials. This structure is produced involving a series of chemical processing steps such as oxidation of silicon, diffusion of impurities into silicon following etching of silicon oxide from selected locations, and deposition and etching of aluminum on silicon to provide connections with the external environment of the transistor. The fabrication process is carried out on a single crystal of silicon available as thin circular wafers of diameter about 30 cm. CMOS technology makes way for two kinds of transistors, namely nMOS (n-channel) transistor and pMOS (p-channel) transistor built by diffusing n-type impurities (rich in electrons) and positively doped silicon (rich in holes) respectively. Some of the distinct layers resulting after the fabrication of a MOS transistor happen to be diffusion, polysilicon and metal (aluminum), separated by insulating layers.

Figure 1.1 depicts the physical structures and circuit symbols of an n-channel and a p-channel transistor. The structure of the n-channel transistor is made of a p-type silicon substrate accommodating two diffused islands of n-type silicon. Selected areas of the p-substrate are altered by diffusion or implantation of n-type impurities. On top of the area separating the n-type islands lies a thin insulating layer of silicon dioxide (SiO₂) above which there is a conducting layer (usually made of polycrystalline silicon) called the gate .