Overview

Objective: Transistors are the main components of digital circuits. Consequently, to truly understand such circuits, it is indispensable to know how transistors are constructed and how they function. Even though the bipolar transistor, which was the first type used in digital circuits, is now used in fewer digital applications, a study of it facilitates an understanding of the evolution of digital logic and why MOS-based circuits are so popular. Moreover, the fastest circuits are still built with bipolar transistors, and new construction techniques have improved their speed even further (these new technologies are also included in this chapter). It is also important to mention that for analog circuits, the bipolar transistor still is a major contender.

Chapter Contents

8.1 Semiconductors

The preferred semiconductor for the fabrication of electronic devices is silicon (Si) because Si-based processes are more mature, and they cost less than other semiconductor processes.

To construct transistors, diodes, or any other devices, the semiconductor must be "doped," which consists of introducing controlled amounts of other materials (called dopants) into the original semiconductor. In the case of Si, which has four valence electrons, such materials belong either to group III (like B, Al, Ga, In) or group V (P, As, Sb) of the periodic table.

Semiconductor doping is illustrated in Figure 8.1. In Figure 8.1(a), a dopant with five valence electrons (phosphorus,...