4.1 INTRODUCTION

The invention of various semiconductor devices and the chronological growth in the field of microelectronics in electronics industry is the backbone of most of the modern electrical/electronic and communication gadgets that have dramatically changed human life, leading to a new technology-driven society. One of the most remarkable breakthroughs to drive technology in the last few decades is the invention of transistor, a three terminal device that is extremely useful and almost an integral part of any analog or digital circuit. The first version of transistor was invented by Dr William Shockley and his co-workers Walter H Brattain and John Bardeen in Bell Lab, USA in 1947. Owing to various operational advantages like low power requirements, high efficiency, etc., coupled with its ultra-compact size, the transistor became very popular since its invention and replaced the previously used vacuum tube-based systems. Over time, the first transistor invented by Shockley et al (point-contact transistor) went through several rounds of modification, resulting in various new variants of the transistor, which include Bipolar Junction Transistor (BJT), Field Effect Transistor (FET), Metal Oxide Semiconductor Field Effect Transistor (MOSFET), etc.

In this chapter, we will discuss the construction of a transistor, basic principle of operation and its two most important applications, namely amplification and switching, in detail. Advanced concept regarding transistor operation and various other functionalities will be discussed in subsequent chapters.

4.2 BJT: CONSTRUCTION AND TERMINALS

A Bipolar Junction Transistor, popularly known as BJT, is a three-terminal semiconductor device with two junctions between them. As shown in Figure 4.1(a), the structure of a transistor resembles a sandwich in which a very narrow and lightly doped p-region is sandwiched between two n-regions. Such a transistor is named as n-p-n transistor. A complementary structure of Figure 4.1(a), as shown in Figure 4.1(b), consists of an n-region sandwiched between two p-regions, resulting in a p-n-p transistor. The term ‘bipolar’ in BJT signifies the presence of two different types of carriers, electron and hole, and the resultant current is the combined contribution of both the carriers. As indicated in Figure 4.1, the three regions of the transistor are named as emitter (E), base (B) and collector (C). They form two junctions: i) emitter-base junction (EBJ), and ii) collector-base junction (CBJ). In a transistor, emitter is heavily doped, whereas base is lightly doped.