Lessons In Electric Circuits Volume III – Semiconductors Book

Chapter 4.1 Introduction

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 176CHAPTER 4. BIPOLAR JUNCTION TRANSISTORS 286,4.16.6 286,Thermal 286,mismatch 286,(problem 286,with 286,paralleling 286,transistors) . . . . . . . . . 277 288,4.16.7 288,High 288,frequency 288,effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 289,Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2804.1IntroductionThe invention of the bipolar transistor in 1948 ushered in a revolution in electronics. Technicalfeats previously requiring relatively large, mechanically fragile, power-hungry vacuum tubeswere suddenly achievable with tiny, mechanically rugged, power-thrifty specks of crystallinesilicon. This revolution made possible the design and manufacture of lightweight, inexpensiveelectronic devices that we now take for granted. Understanding how transistors function is ofparamount importance to anyone interested in understanding modern electronics.My intent here is to focus as exclusively as possible on the practical function and applicationof bipolar transistors, rather than to explore the quantum world of semiconductor theory. Dis-cussions of holes and electrons are better left to another chapter in my opinion. Here I wantto explore how to usethese components, not analyze their intimate internal details. I don’tmean to downplay the importance of understanding semiconductor physics, but sometimesan intense focus on solid-state physics detracts from understanding these devices’ functionson a component level. In taking this approach, however, I assume that the reader possessesa certain minimum knowledge of semiconductors: the difference between “P” and “N” dopedsemiconductors, the functional characteristics of a PN (diode) junction, and the meanings ofthe terms “reverse biased” and “forward biased.” If these concepts are unclear to you, it is bestto refer to earlier chapters in this book before proceeding with this one.A bipolar transistor consists of a three-layer “sandwich” of doped (extrinsic) semiconductormaterials, either P-N-P in Figure 185,4.1(b) or N-P-N at (d). Each layer forming the transistor hasa specific name, and each layer is provided with a wire contact for connection to a circuit. Theschematic symbols are shown in Figure 185,4.1(a) and (d).emitterbasecollectorbaseemittercollectorbaseemittercollectorNPemitterbasecollectorNNPP(a)(b)(c)(d)Figure 4.1:BJT transistor: (a) PNP schematic symbol, (b) physical layout (c) NPN symbol, (d)layout.The functional difference between a PNP transistor and an NPN transistor is the properbiasing (polarity) of the junctions when operating. For any given state of operation, the currentdirections and voltage polarities for each kind of transistor are exactly opposite each other.