Electronic devices-9th-edition-by-floyd

Electron Flow Version

Ninth Edition

Thomas L. Floyd

ELECTRONICDEVICES

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Credits and acknowledgments for materials borrowed from other sources and reproduced, withpermission, in this textbook appear on the appropriate page within text.

Copyright 2012, 2008, 2005, 2002, and 1999 Pearson Education, Inc., publishing asPrentice Hall, 1 Lake Street, Upper Saddle River, New Jersey, 07458. All rights reserved.Manufactured in the United States of America. This publication is protected by Copyright, andpermission should be obtained from the publisher prior to any prohibited reproduction, storage ina retrieval system, or transmission in any form or by any means, electronic, mechanical,photocopying, recording, or likewise. To obtain permission(s) to use material from this work,please submit a written request to Pearson Education, Inc., Permissions Department, 1 LakeStreet, Upper Saddle River, New Jersey 07458.

Library of Congress Cataloging-in-Publication Data

Floyd, Thomas L.Electronic devices : electron flow version / Thomas L. Floyd. 9th ed.

p. cm.Includes index.ISBN-13: 978-0-13-254985-1 (alk. paper)ISBN-10: 0-13-254985-9 (alk. paper)1. Electronic apparatus and appliances. 2. Solid state electronics. I. Title.

TK7870.F52 2012621.3815dc22 2010043463

10 9 8 7 6 5 4 3 2 1

ISBN 10: 0-13-254985-9ISBN 13: 978-0-13-254985-1

PREFACE

This ninth edition of Electronic Devices reflects changes recommended by users andreviewers. Applications and troubleshooting coverage have been expanded to includeseveral new topics related to renewable energy and automated test programming. As in theprevious edition, Chapters 1 through 11 are essentially devoted to discrete devices andcircuits. Chapters 12 through 17 primarily cover linear integrated circuits. A completelynew Chapter 18 covers an introduction to programming for device testing. It can be used asa floating chapter and introduced in conjunction with any of the troubleshootingsections. Chapter 19, which was Chapter 18 in the last edition, is an online chapter thatcovers electronic communications. Multisim files in versions 10 and 11 are now availableat the companion website, www.pearsonhighered.com/electronics.

New in This Edition

Reorganizations of Chapters 1 and 2 These chapters have been significantly re-worked for a more effective coverage of the introduction to electronics and diodes. Newtopics such as the quantum model of the atom have been added.

GreenTech Applications This new feature appears after each of the first six chaptersand introduces the application of electronics to solar energy and wind energy. A significanteffort is being made to create renewable and sustainable energy sources to offset, andeventually replace, fossil fuels. Todays electronics technician should have some familiaritywith these relatively new technologies. The coverage in this text provides a starting pointfor those who may pursue a career in the renewable energy field.

Basic Programming Concepts for Automated Testing A totally new chapter by GarySnyder covers the basics of programming used for the automated testing of electronicdevices. It has become increasingly important for electronic technicians, particularly thoseworking in certain environments such as production testing, to have a fundamental ground-ing in automated testing that involves programming. This chapter is intended to be used inconjunction with the traditional troubleshooting sections and can be introduced or omittedat the instructors discretion.

More Multisim Circuits Updated to Newest Versions Additional Multisim circuitfiles have been added to this edition. All the files have been updated to versions 10 and 11.

New Format for Section Objectives The section objectives have been rewritten to pro-vide a better indication of the coverage in each section. The new format better reflects thetopics covered and their hierarchy.

Miscellaneous Improvements An expanded and updated coverage of LEDs includeshigh-intensity LEDs, which are becoming widely used in many areas such as residentiallighting, automotive lighting, traffic signals, and informational signs. Also, the topic ofquantum dots is discussed, and more emphasis is given to MOSFETs, particularly inswitching power supplies.

www.pearsonhighered.com/electronics

IV PREFACE

Standard Features Full-color format.

Chapter openers include a chapter outline, chapter objectives, introduction, keyterms list, Application Activity preview, and website reference.

Introduction and objectives for each section within a chapter.

Large selection of worked-out examples set off in a graphic box. Each example has arelated problem for which the answer can be found at www.pearsonhighered.com/electronics.

Multisim circuit files for selected examples, troubleshooting, and selected prob-lems are on the companion website.

Section checkup questions are at the end of each section within a chapter. Answerscan be found at www.pearsonhighered.com/electronics.

Troubleshooting sections in many chapters.

An Application Activity is at the end of most chapters.

A Programmable Analog Technology feature is at the end of selected chapters.

A sectionalized chapter summary, key term glossary, and formula list at the end ofeach chapter.

True/false quiz, circuit-action quiz, self-test, and categorized problem set with basicand advanced problems at the end of each chapter.

Appendix with answers to odd-numbered problems, glossary, and index are at theend of the book.

PowerPoint slides, developed by Dave Buchla, are available online. These innova-tive, interactive slides are coordinated with each text chapter and are an excellenttool to supplement classroom presentations.

Student Resources

Companion Website (www.pearsonhighered.com/floyd) This website offers studentsan online study guide that they can check for conceptual understanding of key topics.Also included on the website are the following: Chapter 19, Electronic CommunicationsSystems and Devices, a table of standard resistor values, derivatives of selected equa-tions, a discussion of circuit simulation using Multisim and NI ELVIS, and anexamination of National Instruments LabVIEWTM. The LabVIEW software is an ex-ample of a visual programming application and relates to new Chapter 18. Answers toSection Checkups, Related Problems for Examples, True/False Quizzes, Circuit-Action Quizzes, and Self-Tests are found on this website.

Multisim These online files include simulation circuits in Multisim 10 and 11 forselected examples, troubleshooting sections, and selected problems in the text. Thesecircuits were created for use with Multisim software. Multisim is widely regarded asan excellent circuit simulation tool for classroom and laboratory learning. However, nopart of your textbook is dependent upon the Multisim software or provided files.

Laboratory Exercises for Electronic Devices, Ninth Edition, by Dave Buchla and SteveWetterling. ISBN: 0-13-25419-5.

Instructor Resources

To access supplementary materials online, instructors need to request an instructor access code.Go to www.pearsonhighered.com/irc to register for an instructor access code. Within 48 hoursof registering, you will receive a confirming e-mail including an instructor access code. Onceyou have received your code, locate your text in the online catalog and click on the InstructorResources button on the left side of the catalog product page. Select a supplement, and a login

www.pearsonhighered.com/electronicswww.pearsonhighered.com/electronicswww.pearsonhighered.com/electronicswww.pearsonhighered.com/floydwww.pearsonhighered.com/irc

PREFACE V

page will appear. Once you have logged in, you can access instructor material for all PrenticeHall textbooks. If you have any difficulties accessing the site or downloading a supplement,please contact Customer Service at http://247.prenhall.com.

Online Instructors Resource Manual Includes solutions to chapter problems,Application Activity results, summary of Multisim circuit files, and a test item file.Solutions to the lab manual are also included.

Online Course Support If your program is offering your electronics course in a dis-tance learning format, please contact your local Pearson sales representative for a list ofproduct solutions.

Online PowerPoint Slides This innovative, interactive PowerPoint slide presentationfor each chapter in the book provides an effective supplement to classroom lectures.

Online TestGen This is a test bank of over 800 questions.

Chapter Features

Chapter Opener Each chapter begins with an opening page, as shown in Figure P1. Thechapter opener includes a chapter introduction, a list of chapter sections, chapter objectives,key terms, an Application Activity preview, and a website reference for associated study aids.

2 DIODES AND APPLICATIONSCHAPTER OUTLINE

21 Diode Operation22 Voltage-Current (V-I) Characteristics of a Diode23 Diode Models24 Half-Wave Rectifiers25 Full-Wave Rectifiers26 Power Supply Filters and Regulators27 Diode Limiters and Clampers28 Voltage Multipliers29 The Diode Datasheet210 Troubleshooting

Application ActivityGreenTech Application 2: Solar Power

CHAPTER OBJECTIVES

Use a diode in common applications

Analyze the voltage-current (V-I) characteristic of a diode

Explain how the three diode models differ

Explain and analyze the operation of half-wave rectifiers

Explain and analyze the operation of full-wave rectifiers

Explain and analyze power supply filters and regulators

Explain and analyze the operation of diode limiters andclampers

Explain and analyze the operation of diode voltagemultipliers

Interpret and use diode datasheets

Troubleshoot diodes and power supply circuits

KEY TERMS

VISIT THE COMPANION WEBSITE

Study aids and Multisim files for this chapter are available athttp://www.pearsonhighered.com/electronics

INTRODUCTION

In Chapter 1, you learned that many semiconductor devicesare based on the pn junction. In this chapter, the operationand characteristics of the diode are covered. Also, threediode models representing three levels of approximation arepresented and testing is discussed. The importance of thediode in electronic circuits cannot be overemphasized. Itsability to conduct current in one direction while blockingcurrent in the other direction is essential to the operation ofmany types of circuits. One circuit in particular is the acrectifier, which is covered in this chapter. Other importantapplications are circuits such as diode limiters, diodeclampers, and diode voltage multipliers. A datasheet isdiscussed for specific diodes.

APPLICATION ACTIVITY PREVIEW

You have the responsibility for the final design and testingof a power supply circuit that your company plans to use inseveral of its products. You will apply your knowledge ofdiode circuits to the Application Activity at the end of thechapter.

Diode

Bias

Forward bias

Reverse bias

V-I characteristic

DC power supply

Rectifier

Filter

Regulator

Half-wave rectifier

Peak inverse voltage (PIV)

Full-wave rectifier

Ripple voltage

Line regulation

Load regulation

Limiter

Clamper

Troubleshooting

Section Opener Each section in a chapter begins with a brief introduction and sectionobjectives. An example is shown in Figure P2.

Section Checkup Each section in a chapter ends with a list of questions that focus on themain concepts presented in the section. This feature is also illustrated in Figure P2. Theanswers to the Section Checkups can be found at www.pearsonhighered.com/electronics.

Troubleshooting Sections Many chapters include a troubleshooting section that relatesto the topics covered in the chapter and that illustrates troubleshooting procedures andtechniques. The Troubleshooting section also provides Multisim Troubleshooting exer-cises. A reference to the optional Chapter 18 (Basic Programming Concepts forAutomated Testing) is included in each Troubleshooting section.

List ofperformance-based chapterobjectives

ApplicationActivitypreview

Introduction

Chapter outline

Key terms

Websitereference

FIGURE P1

A typical chapter opener.

http://247.prenhall.comhttp://www.pearsonhighered.com/electronicswww.pearsonhighered.com/electronics

VI PREFACE

482 FET AMPLIFIERS AND SWITCHING CIRCUITS

results in conduction power losses lower than with BJTs. Power MOSFETs are used formotor control, dc-to-ac conversion, dc-to-dc conversion, load switching, and other applica-tions that require high current and precise digital control.

1. Describe a basic CMOS inverter.

2. What type of 2-input digital CMOS circuit has a low output only when both inputs arehigh?

3. What type of 2-input digital CMOS circuit has a high output only when both inputsare low?

SECTION 96 CHECKUP

97 TROUBLESHOOTINGA technician who understands the basics of circuit operation and who can, if necessary,perform basic analysis on a given circuit is much more valuable than one who is limitedto carrying out routine test procedures. In this section, you will see how to test a circuitboard that has only a schematic with no specified test procedure or voltage levels. Inthis case, basic knowledge of how the circuit operates and the ability to do a quickcircuit analysis are useful.

After completing this section, you should be able to

Troubleshoot FET amplifiers Troubleshoot a two-stage common-source amplifier

Explain each step in the troubleshooting procedure Use a datasheet Relate the circuit board to the schematic

A Two-Stage Common-Source Amplifier

Assume that you are given a circuit board containing an audio amplifier and told simplythat it is not working properly. The circuit is a two-stage CS JFET amplifier, as shown inFigure 946.

+12 V

R51.5 k

R6240

C4R410 M

Vout

R21.5 k

R3240

R110 M

Q1Vin Q2

C5

C2100 F 100 F

10 F

0.1 F

C10.1 F

C3

FIGURE 946

A two-stage CS JFET amplifier circuit.

Chapter 18: Basic Programming Concepts for Automated TestingSelected sections from Chapter 18 may be introduced as part of this troubleshootingcoverage or, optionally, the entire Chapter 18 may be covered later or not at all.

FIGURE P2

A typical section opener and sectionreview.

THE COMMON-SOURCE AMPLIFIER 463

The circuit in Figure 914 uses voltage-divider bias to achieve a VGS above threshold.The general dc analysis proceeds as follows using the E-MOSFET characteristic equation(Equation 84) to solve for ID.

The voltage gain expression is the same as for the JFET and D-MOSFET circuits. The acinput resistance is

VDS = VDD - IDRD

ID = K(VGS - VGS(th))2

VGS = a R2R1 + R2 bVDD

Rin R1 ||R2 ||RIN(gate) Equation 95

A common-source amplifier using an E-MOSFET is shown in Figure 917. Find VGS, ID,VDS, and the ac output voltage. Assume that for this particular device, ID(on) 200 mAat VGS 4 V, VGS(th) 2 V, and gm 23 mS. Vin 25 mV.====

=EXAMPLE 98

where RIN(gate) = VGS>IGSS.

VDD+15 V

Vin

C2Vout

RD3.3 k

R2820 k

RL33 k

R14.7 M

10 F

0.01 F

C1

FIGURE 917

Solution

For VGS 4 V,

Therefore,

The ac output voltage is

Related Problem For the E-MOSFET in Figure 917, ID(on) 25 mA at VGS 5 V, VGS(th) 1.5 V,and gm 10 mS. Find VGS, ID, VDS, and the ac output voltage. Vin 25 mV.

Open the Multisim file E09-08 in the Examples folder on the companion website.Determine ID, VDS, and Vout using the specified value of Vin. Compare with thecalculated values.

=====

Vout = AvVin = gmRdVin = (23 mS)(3 k)(25 mV) = 1.73 V

Rd = RD 7 RL = 3.3 k 7 33 k = 3 kVDS = VDD - IDRD = 15 V - (2.65 mA)(3.3 k) = 6.26 VID = K(VGS - VGS(th))2 = (50 mA>V 2)(2.23 V - 2 V)2 = 2.65 mA

K =ID(on)

(VGS - VGS(th))2=

200 mA

(4 V - 2 V)2= 50 mA>V2

VGS = a R2R1 + R2 bVDD = a820 k

5.52 Mb15 V = 2.23 V

FIGURE P3

A typical example with a relatedproblem and Multisim exercise.

Section checkupends eachsection.

Introductoryparagraph beginseach section.

Performance-basedsection objectives

Examples are set off fromtext.

Each example contains arelated problem relevantto the example.

Selected examples include aMultisim exercise coordinatedwith the Multisim circuit fileson the companion website.

Reference to Chapter18, BasicProgrammingConcepts forAutomated Testing

Worked Examples, Related Problems, and Multisim Exercises Numerous worked-out examples throughout each chapter illustrate and clarify basic concepts or specific procedures.Each example ends with a Related Problem that reinforces or expands on the example byrequiring the student to work through a problem similar to the example. Selected examplesfeature a Multisim exercise keyed to a file on the companion website that contains thecircuit illustrated in the example. A typical example with a Related Problem and aMultisim exercise are shown in Figure P3. Answers to Related Problems can be found atwww.pearsonhighered.com/electronics.

www.pearsonhighered.com/electronics

PREFACE VII

Application Activity This feature follows the last section in most chapters and is identi-fied by a special graphic design. A practical application of devices or circuits covered inthe chapter is presented. The student learns how the specific device or circuit is used and istaken through the steps of design specification, simulation, prototyping, circuit boardimplementation, and testing. A typical Application Activity is shown in Figure P4.Application Activities are optional. Results are provided in the Online InstructorsResource Manual.

368 POWER AMPLIFIERS

Application Activity: The Complete PA System

The class AB power amplifier follows the audio preamp and drives the speaker as shown in the PA system block diagram in Figure 734. In this application, the power amplifier isdeveloped and interfaced with the preamp that was developed in Chapter 6. The maximumsignal power to the speaker should be approximately 6 W for a frequency range of 70 Hz to 5 kHz. The dynamic range for the input voltage is up to 40 mV. Finally, the complete PAsystem is put together.

Power amplifier

DC power supplyMicrophone

(a) PA system block diagram (b) Physical configuration

Speaker

Audio preamp

The Power Amplifier Circuit

The schematic of the push-pull power amplifier is shown in Figure 735. The circuit is aclass AB amplifier implemented with Darlington configurations and diode current mirrorbias. Both a traditional Darlington pair and a complementary Darlington (Sziklai) pair areused to provide sufficient current to an speaker load. The signal from the preamp is8

Q4BD135

Q32N3906

Q2BD135

Q12N3904

Input

Output

+15 V

15 V

D1

D2

D3

Q52N3904

R3220

R21 k

R1150 k

FIGURE 735

Class AB power push-pull amplifier.

FIGURE 734

FIGURE P4

Portion of a typical Application Activity section.

372 POWER AMPLIFIERS

Simulate the audio amplifier using your Multisim software. Observe the operationwith the virtual oscilloscope.

Prototyping and Testing

Now that the circuit has been simulated, the prototype circuit is constructed and tested.After the circuit is successfully tested on a protoboard, it is ready to be finalized on aprinted circuit board.

To build and test a similar circuit, go to Experiment 7 in your lab manual (LaboratoryExercises for Electronic Devices by David Buchla and Steven Wetterling).

Circuit Board

The power amplifier is implemented on a printed circuit board as shown in Figure 739.Heat sinks are used to provide additional heat dissipation from the power transistors.

9. Check the printed circuit board and verify that it agrees with the schematic inFigure 735. The volume control potentiometer is mounted off the PC board foreasy access.

10. Label each input and output pin according to function. Locate the single back-side trace.

Heat sink

FIGURE 739

Power amplifier circuit board.

Troubleshooting the Power Amplifier Board

A power amplifier circuit board has failed the production test. Test results are shown inFigure 740.

11. Based on the scope displays, list possible faults for the circuit board.

Putting the System Together

The preamp circuit board and the power amplifier circuit board are interconnected andthe dc power supply (battery pack), microphone, speaker, and volume control poten-tiometer are attached, as shown in Figure 741.

12. Verify that the system interconnections are correct.

Lab Experiment

GreenTech Application Inserts These inserts are placed after each of the first six chap-ters to introduce renewable energy concepts and the application of electronic devices tosolar and wind technologies. Figure P5 illustrates typical GreenTech Application pages.

Chapter End Matter The following pedagogical features are found at the end of mostchapters:

Summary

Key Term Glossary

Key Formulas

True/False Quiz

Circuit-Action Quiz

Self-Test

Basic Problems

Advanced Problems

Datasheet Problems (selected chapters)

Application Activity Problems (many chapters)

Multisim Troubleshooting Problems (most chapters)

Printed circuitboard

Link to experimentin lab manual

Multisim

Activity

Simulationsare providedfor mostApplicationActivity circuits.

VIII PREFACE

Suggestions for Using This Textbook

As mentioned, this book covers discrete devices and circuits in Chapters 1 through 11 andlinear integrated circuits in Chapters 12 through 17. Chapter 18 introduces programmingconcepts for device testing and is linked to Troubleshooting sections.

Option 1 (two terms) Chapters 1 through 11 can be covered in the first term.Depending on individual preferences and program emphasis, selective coverage may benecessary. Chapters 12 through 17 can be covered in the second term. Again, selective cov-erage may be necessary.

Option 2 (one term) By omitting certain topics and by maintaining a rigorous schedule,this book can be used in one-term courses. For example, a course covering only discretedevices and circuits would use Chapters 1 through 11 with, perhaps, some selectivity.

Similarly, a course requiring only linear integrated circuit coverage would use Chapters12 through 17. Another approach is a very selective coverage of discrete devices and circuitstopics followed by a limited coverage of integrated circuits (only op-amps, for example).Also, elements such as the Multisim exercises, Application Activities, and GreenTechApplications can be omitted or selectively used.

To the Student

When studying a particular chapter, study one section until you understand it and only thenmove on to the next one. Read each section and study the related illustrations carefully; thinkabout the material; work through each example step-by-step, work its Related Problem andcheck the answer; then answer each question in the Section Checkup, and check youranswers. Dont expect each concept to be completely clear after a single reading; you mayhave to read the material two or even three times. Once you think that you understand the ma-terial, review the chapter summary, key formula list, and key term definitions at the end of the

In this GreenTech Application, solar tracking is examined. Solar tracking is the process ofmoving the solar panel to track the daily movement of the sun and the seasonal changesin elevation of the sun in the southern sky. The purpose of a solar tracker is to increase theamount of solar energy that can be collected by the system. For flat-panel collectors, anincrease of 30% to 50% in collected energy can be realized with sun tracking comparedto fixed solar panels.

Before looking at methods for tracking, lets review how the sun moves across the sky.The daily motion of the sun follows the arc of a circle from east to west that has its axispointed north near the location of the North Star. As the seasons change from the wintersolstice to the summer solstice, the sun rises a little further to the north each day. Betweenthe summer solstice and the winter solstice, the sun moves further south each day. Theamount of the north-south motion depends on your location.

Single-Axis Solar Tracking

For flat-panel solar collectors, the most economical and generally most practical solutionto tracking is to follow the daily east-west motion, and not the annual north-south motion.The daily east-to-west motion can be followed with a single-axis tracking system. Thereare two basic single-axis systems: polar and azimuth. In a polar system, the main axis ispointed to the polar north (North Star), as shown in Figure GA41(a). (In telescopeterminology, this is called an equatorial mounting.) The advantage is that the solar panel iskept at an angle facing the sun at all times because it tracks the sun from east to west andis angled toward the southern sky. In an azimuth tracking system, the motor drives thesolar panel and frequently multiple panels. The panels can be oriented horizontally butstill track the east-to-west motion of the sun. Although this does not intercept as much ofthe sunlight during the seasons, it has less wind loading and is more feasible for longrows of solar panels. Figure GA41(b) shows a solar array that is oriented horizontallywith the axis pointing to true north and uses azimuth tracking (east to west). As you cansee, sunlight will strike the polar-aligned panel more directly during the seasonal movementof the sun than it will with the horizontal orientation of the azimuth tracker.

GreenTech Application 4: Solar Power

(a) A single-axis polar-aligned tracker

EastWest

(North Star)

Polar North

East

(b) Single-axis azimuth tracker

West

Electricmotorturns thepanels True North

224 BIPOLAR JUNCTION TRANSISTORS

FIGURE GA41

Types of single-axis solar tracking.

Some solar tracking systems combine both the azimuth and the elevation tracking, whichis known as dual-axis tracking. Ideally, the solar panel should always face directlytoward the sun so that the sun light rays are perpendicular to the panel. With dual-axistracking, the annual north-south motion of the sun can be followed in addition to the

daily east-to-west movement. This is particularly important with concentrating collectorsthat need to be oriented correctly to focus the sun on the active region.

Figure GA42 is an example showing the improvement in energy collection of a typicaltracking panel versus a nontracking panel for a flat solar collector. As you can see, track-ing extends the time that a given output can be maintained.

There are several methods of implementing solar tracking. Two main ones are sensor con-trolled and timer controlled.

Sensor-Controlled Solar Tracking

This type of tracking control uses photosensitive devices such as photodiodes or photo-resistors. Typically, there are two light sensors for the azimuth control and two for the ele-vation control. Each pair senses the direction of light from the sun and activates the motorcontrol to move the solar panel to align perpendicular to the suns rays.

Figure GA43 shows the basic idea of a sensor-controlled tracker. Two photodiodes witha light-blocking partition between them are mounted on the same plane as the solar panel.

GREENTECH APPLICATION 4 225

FIGURE GA42

Graphs of voltages in tracking andnontracking (fixed) solar panels.

Photodiodes

(a) Outputs of the photodiodes are unequal if solar panel is not directly facing the sun.

(b) Outputs of the photodiodes are equal when solar panel orientation is optimum.

Solar panel

SUN

Lower output Higher output

Output rotates motor

Position controlcircuits

SUN

FIGURE GA43

Simplified illustration of a light-sensing control for a solar-tracking system. Relative sizes are exagger-ated to demonstrate the concept.

Relative output voltage

Time of day

Panels rated current

6 7 8 9 10 11 12 1 2 3 4 5 6 7

Tracking

Nontracking

FIGURE P5

Portion of a typical GreenTech Application.

PREFACE IX

chapter. Take the true/false quiz, the circuit-action quiz, and the self-test. Finally, work the as-signed problems at the end of the chapter. Working through these problems is perhaps themost important way to check and reinforce your comprehension of the chapter. By workingproblems, you acquire an additional level of insight and understanding, and develop logicalthinking that reading or classroom lectures alone do not provide.

Generally, you cannot fully understand a concept or procedure by simply watching orlistening to someone else. Only hard work and critical thinking will produce the resultsyou expect and deserve.

Acknowledgments

Many capable people have contributed to the ninth edition of Electronic Devices. It hasbeen thoroughly reviewed and checked for both content and accuracy. Those at PrenticeHall who have contributed greatly to this project throughout the many phases of develop-ment and production include Rex Davidson, Yvette Schlarman, and Wyatt Morris. LoisPorter has once more done an outstanding job editing the manuscript. Thanks to SudipSinha at Aptara for his management of the art and text programs. Dave Buchla contributedextensively to the content of the book, helping to make this edition the best one yet. GarySnyder created the circuit files for the Multisim features in this edition. Gary also wroteChapter 18, Basic Programming Concepts for Automated Testing. I wish to express myappreciation to those already mentioned as well as the reviewers who provided many valu-able suggestions and constructive criticism that greatly influenced this edition. Thesereviewers are William Dolan, Kennebec Valley Community College; John Duncan, KentState University; Art Eggers, Community College of Southern Nevada; Paul Garrett, ITTTechnical Institute; Mark Hughes, Cleveland Community College; Lisa Jones, SouthwestTennessee Community College; Max Rabiee, University of Cincinnati; and Jim Rhodes,Blue Ridge Community College.

Tom Floyd

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BRIEF CONTENTS

11 Thyristors 564

12 The Operational Amplifier 602

13 Basic Op-Amp Circuits 667

14 Special-Purpose Op-Amp Circuits 718

15 Active Filters 763

16 Oscillators 806

17 Voltage Regulators 851

18 Basic Programming Concepts for Automated Testing 890

Answers to Odd-Numbered Problems 931

Glossary 944

Index 951

1 Introduction to Electronics 1

2 Diodes and Applications 30

3 Special-Purpose Diodes 112

4 Bipolar Junction Transistors 173

5 Transistor Bias Circuits 228

6 BJT Amplifiers 271

7 Power Amplifiers 339

8 Field-Effect Transistors (FETs) 384

9 FET Amplifiers and Switching Circuits 451

10 Amplifier Frequency Response 505

CONTENTS

1 Introduction to Electronics 111 The Atom 212 Materials Used in Electronics 713 Current in Semiconductors 1114 N-Type and P-Type Semiconductors 1415 The PN Junction 16

GreenTech Application 1: Solar Power 24

2 Diodes and Applications 3021 Diode Operation 3122 Voltage-Current (V-I) Characteristics 3623 Diode Models 3924 Half-Wave Rectifiers 4425 Full-Wave Rectifiers 5026 Power Supply Filters and Regulators 5727 Diode Limiters and Clampers 6428 Voltage Multipliers 7129 The Diode Datasheet 73210 Troubleshooting 76

Application Activity 85GreenTech Application 2: Solar Power 108

3 Special-Purpose Diodes 11231 The Zener Diode 11332 Zener Diode Applications 12033 The Varactor Diode 12834 Optical Diodes 13335 Other Types of Diodes 14736 Troubleshooting 153


4 Bipolar Junction Transistors 17341 Bipolar Junction Transistor (BJT) Structure 17442 Basic BJT Operation 17543 BJT Characteristics and Parameters 17744 The BJT as an Amplifier 19045 The BJT as a Switch 19246 The Phototransistor 196

47 Transistor Categories and Packaging 19948 Troubleshooting 201


5 Transistor Bias Circuits 22851 The DC Operating Point 22952 Voltage-Divider Bias 23553 Other Bias Methods 24154 Troubleshooting 248

Application Activity 252GreenTech Application 5: Wind Power 267

6 BJT Amplifiers 27161 Amplifier Operation 27262 Transistor AC Models 27563 The Common-Emitter Amplifier 27864 The Common-Collector Amplifier 29165 The Common-Base Amplifier 29866 Multistage Amplifiers 30167 The Differential Amplifier 30468 Troubleshooting 310

Application Activity 314GreenTech Application 6: Wind Power 335

7 Power Amplifiers 33971 The Class A Power Amplifier 34072 The Class B and Class AB Push-Pull

Amplifiers 34673 The Class C Amplifier 35774 Troubleshooting 365

Application Activity 368

8 Field-Effect Transistors (FETs) 38481 The JFET 38582 JFET Characteristics and Parameters 38783 JFET Biasing 39784 The Ohmic Region 40885 The MOSFET 412

XIV CONTENTS

86 MOSFET Characteristics and Parameters 41787 MOSFET Biasing 42088 The IGBT 42389 Troubleshooting 425


9 FET Amplifiers and Switching Circuits 45191 The Common-Source Amplifier 45292 The Common-Drain Amplifier 46493 The Common-Gate Amplifier 46794 The Class D Amplifier 47095 MOSFET Analog Switching 47496 MOSFET Digital Switching 47997 Troubleshooting 482


10 Amplifier Frequency Response 505101 Basic Concepts 506102 The Decibel 509103 Low-Frequency Amplifier Response 512104 High-Frequency Amplifier Response 530105 Total Amplifier Frequency Response 540106 Frequency Response of Multistage Amplifiers 543107 Frequency Response Measurements 546


11 Thyristors 564111 The Four-Layer Diode 565112 The Silicon-Controlled Rectifier (SCR) 568113 SCR Applications 573114 The Diac and Triac 578115 The Silicon-Controlled Switch (SCS) 582116 The Unijunction Transistor (UJT) 583117 The Programmable Unijunction

Transistor (PUT) 588Application Activity 590

12 The Operational Amplifier 602121 Introduction to Operational Amplifiers 603122 Op-Amp Input Modes and Parameters 605123 Negative Feedback 613124 Op-Amps with Negative Feedback 614125 Effects of Negative Feedback on Op-Amp

Impedances 619126 Bias Current and Offset Voltage 624127 Open-Loop Frequency and Phase Responses 627128 Closed-Loop Frequency Response 633129 Troubleshooting 636

Application Activity 638Programmable Analog Technology 644

13 Basic Op-Amp Circuits 667131 Comparators 668132 Summing Amplifiers 679133 Integrators and Differentiators 687134 Troubleshooting 694


14 Special-Purpose Op-Amp Circuits 718141 Instrumentation Amplifiers 719142 Isolation Amplifiers 725143 Operational Transconductance

Amplifiers (OTAs) 730144 Log and Antilog Amplifiers 736145 Converters and Other Op-Amp Circuits 742


15 Active Filters 763151 Basic Filter Responses 764152 Filter Response Characteristics 768153 Active Low-Pass Filters 772154 Active High-Pass Filters 776155 Active Band-Pass Filters 779156 Active Band-Stop Filters 785157 Filter Response Measurements 787


16 Oscillators 806161 The Oscillator 807162 Feedback Oscillators 808163 Oscillators with RC Feedback Circuits 810164 Oscillators with LC Feedback Circuits 817165 Relaxation Oscillators 825166 The 555 Timer as an Oscillator 830


17 Voltage Regulators 851171 Voltage Regulation 852172 Basic Linear Series Regulators 855173 Basic Linear Shunt Regulators 860174 Basic Switching Regulators 863175 Integrated Circuit Voltage Regulators 869176 Integrated Circuit Voltage Regulator

Configurations 875Application Activity 879

CONTENTS XV

18 Basic Programming Concepts for Automated Testing 890

181 Programming Basics 891182 Automated Testing Basics 893183 The Simple Sequential Program 898184 Conditional Execution 900185 Program Loops 905186 Branching and Subroutines 913

Answers to Odd-Numbered Problems 931

Glossary 944

Index 951

1INTRODUCTION TOELECTRONICSCHAPTER OUTLINE

11 The Atom12 Materials Used in Electronics13 Current in Semiconductors14 N-Type and P-Type Semiconductors15 The PN Junction

GreenTech Application 1: Solar Power

CHAPTER OBJECTIVES

Describe the structure of an atom

Discuss insulators, conductors, and semiconductors andhow they differ

Describe how current is produced in a semiconductor

Describe the properties of n-type and p-type semiconductors

Describe how a pn junction is formed

KEY TERMS

VISIT THE COMPANION WEBSITE

Study aids for this chapter are available athttp://www.pearsonhighered.com/electronics

INTRODUCTION

Electronic devices such as diodes, transistors, and integratedcircuits are made of a semiconductive material. To under-stand how these devices work, you should have a basicknowledge of the structure of atoms and the interaction ofatomic particles. An important concept introduced in thischapter is that of the pn junction that is formed when twodifferent types of semiconductive material are joined. The pnjunction is fundamental to the operation of devices such asthe solar cell, the diode, and certain types of transistors.

Atom

Proton

Electron

Shell

Valence

Ionization

Free electron

Orbital

Insulator

Conductor

Semiconductor

Silicon

Crystal

Hole

Doping

PN junction

Barrier potential

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2 INTRODUCTION TO ELECTRONICS

The Bohr Model

An atom* is the smallest particle of an element that retains the characteristics of that ele-ment. Each of the known 118 elements has atoms that are different from the atoms of allother elements. This gives each element a unique atomic structure. According to the clas-sical Bohr model, atoms have a planetary type of structure that consists of a central nucleussurrounded by orbiting electrons, as illustrated in Figure 11. The nucleus consists of pos-itively charged particles called protons and uncharged particles called neutrons. Thebasic particles of negative charge are called electrons.

Each type of atom has a certain number of electrons and protons that distinguishes itfrom the atoms of all other elements. For example, the simplest atom is that of hydrogen,which has one proton and one electron, as shown in Figure 12(a). As another example, thehelium atom, shown in Figure 12(b), has two protons and two neutrons in the nucleus andtwo electrons orbiting the nucleus.

Atomic Number

All elements are arranged in the periodic table of the elements in order according to theiratomic number. The atomic number equals the number of protons in the nucleus, which isthe same as the number of electrons in an electrically balanced (neutral) atom. For example,hydrogen has an atomic number of 1 and helium has an atomic number of 2. In their normal(or neutral) state, all atoms of a given element have the same number of electrons as protons;the positive charges cancel the negative charges, and the atom has a net charge of zero.

*All bold terms are in the end-of-book glossary. The bold terms in color are key terms and are also definedat the end of the chapter.

11 THE ATOMAll matter is composed of atoms; all atoms consist of electrons, protons, and neutronsexcept normal hydrogen, which does not have a neutron. Each element in the periodictable has a unique atomic structure, and all atoms within a given element have the samenumber of protons. At first, the atom was thought to be a tiny indivisible sphere. Later itwas shown that the atom was not a single particle but was made up of a small densenucleus around which electrons orbit at great distances from the nucleus, similar to theway planets orbit the sun. Niels Bohr proposed that the electrons in an atom circle thenucleus in different obits, similar to the way planets orbit the sun in our solar system. TheBohr model is often referred to as the planetary model. Another view of the atom calledthe quantum model is considered a more accurate representation, but it is difficult tovisualize. For most practical purposes in electronics, the Bohr model suffices and iscommonly used because it is easy to visualize.


Describe the structure of an atom Discuss the Bohr model of an atom Define electron, proton, neutron, andnucleus

Define atomic number Discuss electron shells and orbits

Explain energy levels Define valence electron Discuss ionization

Define free electron and ion Discuss the basic concept of the quantum model of the atom

Niels Henrik David Bohr (October 7,1885November 18, 1962) was aDanish physicist, who madeimportant contributions tounderstanding the structure of theatom and quantum mechanics bypostulating the planetary modelof the atom. He received the Nobelprize in physics in 1922. Bohr drewupon the work or collaboratedwith scientists such as Dalton,Thomson, and Rutherford, amongothers and has been described asone of the most influentialphysicists of the 20th century.

H I S T O R Y N O T E

THE ATOM 3

Electron Proton Neutron

FIGURE 11

The Bohr model of an atom showing electrons in orbits around the nucleus, which consists ofprotons and neutrons. The tails on the electrons indicate motion.

(a) Hydrogen atom (b) Helium atom

Electron

Nucleus

Electron

Nucleus

Electron

FIGURE 12

Two simple atoms, hydrogen and helium.

Atomic numbers of all the elements are shown on the periodic table of the elements inFigure 13.

Electrons and Shells

Energy Levels Electrons orbit the nucleus of an atom at certain distances from the nu-cleus. Electrons near the nucleus have less energy than those in more distant orbits. Onlydiscrete (separate and distinct) values of electron energies exist within atomic structures.Therefore, electrons must orbit only at discrete distances from the nucleus.

Each discrete distance (orbit) from the nucleus corresponds to a certain energy level. Inan atom, the orbits are grouped into energy levels known as shells. A given atom has afixed number of shells. Each shell has a fixed maximum number of electrons. The shells(energy levels) are designated 1, 2, 3, and so on, with 1 being closest to the nucleus. TheBohr model of the silicon atom is shown in Figure 14. Notice that there are 14 electronsand 14 each of protons and neutrons in the nucleus.


1

H

SiliconAtomic number = 14

HeliumAtomic number = 2

2

He

3

Li4

Be

11

Na12

Mg

71

Lu58

Ce60

Nd61

Pm62

Sm63

Eu59

Pr57

La64

Gd65

Tb67

Ho68

Er69

Tm70

Yb66

Dy

103

Lr90

Th89

Ac92

U93

Np94

Pu95

Am91

Pa96

Cm97

Bk99

Es100

Fm101

Md102

No98

Cf

19

K20

Ca21

Sc22

Ti23

V24

Cr25

Mn26

Fe27

Co28

Ni29

Cu30

Zn31

Ga32

Ge33

As34

Se35

Br36

Kr

37

Rb38

Sr39

Y40

Zr41

Nb42

Mo43

Tc44

Ru45

Rh46

Pd47

Ag48

Cd49

In50

Sn51

Sb52

Te53

I54

Xe

55

Cs56

Ba72

Hf73

Ta74

W75

Re76

Os77

Ir* 78

Pt79

Au80

Hg81

Tl82

Pb83

Bi84

Po85

At86

Rn

5

B6

C7

N8

O9

F10

Ne

13

Al14

Si15

P16

S17

Cl18

Ar

87

Fr88

Ra104

Rf105

Db106

Sg107

Bh108

Hs109

Mt** 110 111 112 114113 115 116

Ds Rg Cp Uut Uuq Uup Uuh117 118Uus Uuo

FIGURE 13

The periodic table of the elements. Some tables also show atomic mass.

Shell 1Shell 2Shell 3

Nucleus14p, 14n

FIGURE 14

Illustration of the Bohr model of thesilicon atom.

where n is the number of the shell. The maximum number of electrons that can exist in theinnermost shell (shell 1) is

Ne = 2n2 = 2(1)2 = 2

Ne 2n2Equation 11

The Maximum Number of Electrons in Each Shell The maximum number of elec-trons (Ne) that can exist in each shell of an atom is a fact of nature and can be calculated bythe formula,

THE ATOM 5

The maximum number of electrons that can exist in shell 2 is



Valence Electrons

Electrons that are in orbits farther from the nucleus have higher energy and are less tightlybound to the atom than those closer to the nucleus. This is because the force of attractionbetween the positively charged nucleus and the negatively charged electron decreases withincreasing distance from the nucleus. Electrons with the highest energy exist in the outer-most shell of an atom and are relatively loosely bound to the atom. This outermost shell isknown as the valence shell and electrons in this shell are called valence electrons. Thesevalence electrons contribute to chemical reactions and bonding within the structure of amaterial and determine its electrical properties. When a valence electron gains sufficientenergy from an external source, it can break free from its atom. This is the basis for con-duction in materials.

Ionization

When an atom absorbs energy from a heat source or from light, for example, the energiesof the electrons are raised. The valence electrons possess more energy and are moreloosely bound to the atom than inner electrons, so they can easily jump to higher energyshells when external energy is absorbed by the atom.

If a valence electron acquires a sufficient amount of energy, called ionization energy, itcan actually escape from the outer shell and the atoms influence. The departure of a valenceelectron leaves a previously neutral atom with an excess of positive charge (more protonsthan electrons). The process of losing a valence electron is known as ionization, and theresulting positively charged atom is called a positive ion. For example, the chemical symbolfor hydrogen is H. When a neutral hydrogen atom loses its valence electron and becomes apositive ion, it is designated H. The escaped valence electron is called a free electron.

The reverse process can occur in certain atoms when a free electron collides with the atomand is captured, releasing energy. The atom that has acquired the extra electron is called anegative ion. The ionization process is not restricted to single atoms. In many chemical reac-tions, a group of atoms that are bonded together can lose or acquire one or more electrons.

For some nonmetallic materials such as chlorine, a free electron can be captured by theneutral atom, forming a negative ion. In the case of chlorine, the ion is more stable than theneutral atom because it has a filled outer shell. The chlorine ion is designated as

The Quantum Model

Although the Bohr model of an atom is widely used because of its simplicity and ease ofvisualization, it is not a complete model. The quantum model, a more recent model, is con-sidered to be more accurate. The quantum model is a statistical model and very difficult tounderstand or visualize. Like the Bohr model, the quantum model has a nucleus of protonsand neutrons surrounded by electrons. Unlike the Bohr model, the electrons in the quan-tum model do not exist in precise circular orbits as particles. Two important theories under-lie the quantum model: the wave-particle duality and the uncertainty principle.

Wave-particle duality. Just as light can be both a wave and a particle (photon),electrons are thought to exhibit a dual characteristic. The velocity of an orbiting elec-tron is considered to be its wavelength, which interferes with neighboring electronwaves by amplifying or canceling each other.

Cl-.

Ne = 2n2 = 2(4)2 = 2(16) = 32

Ne = 2n2 = 2(3)2 = 2(9) = 18

Ne = 2n2 = 2(2)2 = 2(4) = 8 Atoms are extremely small andcannot be seen even with thestrongest optical microscopes;however, a scanning tunnelingmicroscope can detect a singleatom. The nucleus is so small andthe electrons orbit at suchdistances that the atom is mostlyempty space. To put it inperspective, if the proton in ahydrogen atom were the size of agolf ball, the electron orbit wouldbe approximately one mile away.

Protons and neutrons areapproximately the same mass. Themass of an electron is 1 1836 of aproton. Within protons andneutrons there are even smallerparticles called quarks.

>

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Uncertainly principle. As you know, a wave is characterized by peaks and valleys;therefore, electrons acting as waves cannot be precisely identified in terms of their posi-tion. According to Heisenberg, it is impossible to determine simultaneously both theposition and velocity of an electron with any degree of accuracy or certainty. The resultof this principle produces a concept of the atom with probability clouds, which aremathematical descriptions of where electrons in an atom are most likely to be located.

In the quantum model, each shell or energy level consists of up to four subshells calledorbitals, which are designated s, p, d, and f. Orbital s can hold a maximum of two electrons,orbital p can hold six electrons, orbital d can hold ten electrons, and orbital f can hold four-teen electrons. Each atom can be described by an electron configuration table that showsthe shells or energy levels, the orbitals, and the number of electrons in each orbital. Forexample, the electron configuration table for the nitrogen atom is given in Table 11. Thefirst full-size number is the shell or energy level, the letter is the orbital, and the exponentis the number of electrons in the orbital.

De Broglie showed that everyparticle has wave characteristics.Schrodiger developed a waveequation for electrons.

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TABLE 11

Electron configuration table for nitrogen.

NOTATION EXPLANATION

1s2 2 electrons in shell 1, orbital s

2s2 2p3 5 electrons in shell 2: 2 in orbital s, 3 in orbital p

TABLE 12 NOTATION EXPLANATION

1s2 2 electrons in shell 1, orbital s



Atomic orbitals do not resemble a discrete circular path for the electron as depicted inBohrs planetary model. In the quantum picture, each shell in the Bohr model is a three-dimensional space surrounding the atom that represents the mean (average) energy of theelectron cloud. The term electron cloud (probability cloud) is used to describe the areaaround an atoms nucleus where an electron will probably be found.

Using the atomic number from the periodic table in Figure 13, describe a silicon (Si)atom using an electron configuration table.

Solution The atomic number of silicon is 14. This means that there are 14 protons in the nucleus.Since there is always the same number of electrons as protons in a neutral atom, thereare also 14 electrons. As you know, there can be up to two electrons in shell 1, eight inshell 2, and eighteen in shell 3. Therefore, in silicon there are two electrons in shell 1,eight electrons in shell 2, and four electrons in shell 3 for a total of 14 electrons. Theelectron configuration table for silicon is shown in Table 12.

EXAMPLE 11

Related Problem* Develop an electron configuration table for the germanium (Ge) atom in the periodic table.

In a three-dimensional representation of the quantum model of an atom, the s-orbitalsare shaped like spheres with the nucleus in the center. For energy level 1, the sphere issolid but for energy levels 2 or more, each single s-orbital is composed of spherical surfacesthat are nested shells. A p-orbital for shell 2 has the form of two ellipsoidal lobes with apoint of tangency at the nucleus (sometimes referred to as a dumbbell shape.) The three

*Answers can be found at www.pearsonhighered.com/floyd.

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MATERIALS USED IN ELECTRONICS 7

p-orbitals in each energy level are oriented at right angles to each other. One is oriented onthe x-axis, one on the y-axis, and one on the z-axis. For example, a view of the quantummodel of a sodium atom (Na) that has 11 electrons is shown in Figure 15. The three axesare shown to give you a 3-D perspective.

2pz orbital (2 electrons)

2px orbital (2 electrons)

2py orbital (2 electrons)

1s orbital (2 electrons)

2s orbital (2 electrons)3s orbital (1 electron)

Nucleus

x-axis

z-axis

y-axis

FIGURE 15

Three-dimensional quantum modelof the sodium atom, showing the orbitals and number of electrons ineach orbital.

1. Describe the Bohr model of the atom.

2. Define electron.

3. What is the nucleus of an atom composed of? Define each component.

4. Define atomic number.

5. Discuss electron shells and orbits and their energy levels.

6. What is a valence electron?

7. What is a free electron?

8. Discuss the difference between positive and negative ionization.

9. Name two theories that distinguish the quantum model.

SECTION 11 CHECKUPAnswers can be found at www.pearsonhighered.com/floyd.

12 MATERIALS USED IN ELECTRONICSIn terms of their electrical properties, materials can be classified into three groups: con-ductors, semiconductors, and insulators. When atoms combine to form a solid, crystallinematerial, they arrange themselves in a symmetrical pattern. The atoms within the crystalstructure are held together by covalent bonds, which are created by the interaction of thevalence electrons of the atoms. Silicon is a crystalline material.


Discuss insulators, conductors, and semiconductors and how they differ Define the core of an atom Describe the carbon atom Name two typeseach of semiconductors, conductors, and insulators

Explain the band gap Define valence band and conduction band Compare a semiconductor atomto a conductor atom

Discuss silicon and gemanium atoms Explain covalent bonds

Define crystal

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Insulators, Conductors, and Semiconductors

All materials are made up of atoms. These atoms contribute to the electrical properties of amaterial, including its ability to conduct electrical current.

For purposes of discussing electrical properties, an atom can be represented by thevalence shell and a core that consists of all the inner shells and the nucleus. This concept isillustrated in Figure 16 for a carbon atom. Carbon is used in some types of electricalresistors. Notice that the carbon atom has four electrons in the valence shell and two electronsin the inner shell. The nucleus consists of six protons and six neutrons, so the 6 indicatesthe positive charge of the six protons. The core has a net charge of 4 (6 for the nucleusand for the two inner-shell electrons).

Insulators An insulator is a material that does not conduct electrical current under nor-mal conditions. Most good insulators are compounds rather than single-element materialsand have very high resistivities. Valence electrons are tightly bound to the atoms; there-fore, there are very few free electrons in an insulator. Examples of insulators are rubber,plastics, glass, mica, and quartz.

Conductors A conductor is a material that easily conducts electrical current. Mostmetals are good conductors. The best conductors are single-element materials, such ascopper (Cu), silver (Ag), gold (Au), and aluminum (Al), which are characterized by atomswith only one valence electron very loosely bound to the atom. These loosely bound va-lence electrons become free electrons. Therefore, in a conductive material the free elec-trons are valence electrons.

Semiconductors A semiconductor is a material that is between conductors and insula-tors in its ability to conduct electrical current. A semiconductor in its pure (intrinsic) stateis neither a good conductor nor a good insulator. Single-element semiconductors areantimony (Sb), arsenic (As), astatine (At), boron (B), polonium (Po), tellurium (Te),silicon (Si), and germanium (Ge). Compound semiconductors such as gallium arsenide,indium phosphide, gallium nitride, silicon carbide, and silicon germanium are also com-monly used. The single-element semiconductors are characterized by atoms with four va-lence electrons. Silicon is the most commonly used semiconductor.

Band Gap

Recall that the valence shell of an atom represents a band of energy levels and that the va-lence electrons are confined to that band. When an electron acquires enough additional en-ergy, it can leave the valence shell, become a free electron, and exist in what is known asthe conduction band.

The difference in energy between the valence band and the conduction band is calledan energy gap or band gap. This is the amount of energy that a valence electron musthave in order to jump from the valence band to the conduction band. Once in the conduc-tion band, the electron is free to move throughout the material and is not tied to anygiven atom.

Figure 17 shows energy diagrams for insulators, semiconductors, and conductors. Theenergy gap or band gap is the difference between two energy levels and is not allowed inquantum theory. It is a region in insulators and semiconductors where no electron statesexist. Although an electron may not exist in this region, it can jump across it under cer-tain conditions. For insulators, the gap can be crossed only when breakdown conditionsoccuras when a very high voltage is applied across the material. The band gap is illus-trated in Figure 17(a) for insulators. In semiconductors the band gap is smaller, allowingan electron in the valence band to jump into the conduction band if it absorbs a photon. Theband gap depends on the semiconductor material. This is illustrated in Figure 17(b). Inconductors, the conduction band and valence band overlap, so there is no gap, as shown inFigure 17(c). This means that electrons in the valence band move freely into the conduc-tion band, so there are always electrons available as free electrons.

-2

Core (+4)

Valence electrons

+6

FIGURE 16

Diagram of a carbon atom.

Next to silicon, the second mostcommon semiconductive materialis gallium arsenide, GaAs. This is acrystalline compound, not anelement. Its properties can becontrolled by varying the relativeamount of gallium and arsenic.

GaAs has the advantage ofmaking semiconductor devices thatrespond very quickly to electricalsignals. This makes it better thansilicon for applications likeamplifying the high frequency (1 GHz to 10 GHz) signals from TVsatellites, etc. The maindisadvantage of GaAs is that it ismore difficult to make and thechemicals involved are quite oftentoxic!

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MATERIALS USED IN ELECTRONICS 9

Comparison of a Semiconductor Atom to a Conductor Atom

Silicon is a semiconductor and copper is a conductor. Bohr diagrams of the silicon atom andthe copper atom are shown in Figure 18. Notice that the core of the silicon atom has a netcharge of 4 (14 protons 10 electrons) and the core of the copper atom has a net charge of1 (29 protons 28 electrons). The core includes everything except the valence electrons.

Conduction band

Energy Energy Energy

Valence band

Conduction band

Valence band

Conduction band

Valence band

0 0 0

(c) Conductor(b) Semiconductor(a) Insulator

Overlap

Band gap

Band gap

FIGURE 17

Energy diagrams for the three typesof materials.

The valence electron in the copper atom feels an attractive force of 1 compared to avalence electron in the silicon atom which feels an attractive force of 4. Therefore,there is more force trying to hold a valence electron to the atom in silicon than in copper.The coppers valence electron is in the fourth shell, which is a greater distance from its nu-cleus than the silicons valence electron in the third shell. Recall that electrons farthestfrom the nucleus have the most energy. The valence electron in copper has more energythan the valence electron in silicon. This means that it is easier for valence electrons incopper to acquire enough additional energy to escape from their atoms and become freeelectrons than it is in silicon. In fact, large numbers of valence electrons in copper alreadyhave sufficient energy to be free electrons at normal room temperature.

Silicon and Germanium

The atomic structures of silicon and germanium are compared in Figure 19. Silicon isused in diodes, transistors, integrated circuits, and other semiconductor devices. Noticethat both silicon and germanium have the characteristic four valence electrons.

(b) Copper atom(a) Silicon atom

Core (+4)

Core (+1)Valence electrons

Valence electron

+14

+29

FIGURE 18

Bohr diagrams of the silicon andcopper atoms.


Germanium atom

+14

Silicon atom

Four valence electrons inthe outer (valence) shell

+32

FIGURE 19

Diagrams of the silicon and germa-nium atoms.

(a) (b) Bonding diagram. The red negative signs represent the shared valence electrons.

The center silicon atom shares an electron with eachof the four surrounding silicon atoms, creating acovalent bond with each. The surrounding atoms arein turn bonded to other atoms, and so on.

Si

SiSiSi

Si

+4

+4

+4

+4

+4

FIGURE 110

Illustration of covalent bonds in silicon.

The valence electrons in germanium are in the fourth shell while those in silicon are inthe third shell, closer to the nucleus. This means that the germanium valence electrons areat higher energy levels than those in silicon and, therefore, require a smaller amount of ad-ditional energy to escape from the atom. This property makes germanium more unstable athigh temperatures and results in excessive reverse current. This is why silicon is a morewidely used semiconductive material.

Covalent Bonds Figure 110 shows how each silicon atom positions itself with fouradjacent silicon atoms to form a silicon crystal. A silicon (Si) atom with its four valenceelectrons shares an electron with each of its four neighbors. This effectively creates eightshared valence electrons for each atom and produces a state of chemical stability. Also, thissharing of valence electrons produces the covalent bonds that hold the atoms together;each valence electron is attracted equally by the two adjacent atoms which share it.Covalent bonding in an intrinsic silicon crystal is shown in Figure 111. An intrinsic crys-tal is one that has no impurities. Covalent bonding for germanium is similar because it alsohas four valence electrons.

CURRENT IN SEMICONDUCTORS 11

Si SiSi Si Si

Si SiSi Si Si

Si SiSi Si Si

Si SiSi Si Si

FIGURE 111

Covalent bonds in a silicon crystal.

1. What is the basic difference between conductors and insulators?

2. How do semiconductors differ from conductors and insulators?

3. How many valence electrons does a conductor such as copper have?

4. How many valence electrons does a semiconductor have?

5. Name three of the best conductive materials.

6. What is the most widely used semiconductive material?

7. Why does a semiconductor have fewer free electrons than a conductor?

8. How are covalent bonds formed?

9. What is meant by the term intrinsic?

10. What is a crystal?

SECTION 12 CHECKUP

13 CURRENT IN SEMICONDUCTORSThe way a material conducts electrical current is important in understanding howelectronic devices operate. You cant really understand the operation of a device suchas a diode or transistor without knowing something about current in semiconductors.


Describe how current is produced in a semiconductor Discuss conduction electrons and holes

Explain an electron-hole pair Discuss recombination Explain electron and hole current

As you have learned, the electrons of an atom can exist only within prescribed energybands. Each shell around the nucleus corresponds to a certain energy band and is separatedfrom adjacent shells by band gaps, in which no electrons can exist. Figure 112 shows theenergy band diagram for an unexcited (no external energy such as heat) atom in a pure sil-icon crystal. This condition occurs only at a temperature of absolute 0 Kelvin.


Conduction Electrons and Holes

An intrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal) energyfor some valence electrons to jump the gap from the valence band into the conduction band,becoming free electrons. Free electrons are also called conduction electrons. This is illus-trated in the energy diagram of Figure 113(a) and in the bonding diagram of Figure 113(b).

Energy

Band gap

Second band (shell 2)

First band (shell 1)

Nucleus

Valence band (shell 3)

Conduction band

FIGURE 112

Energy band diagram for an unexcitedatom in a pure (intrinsic) siliconcrystal. There are no electrons in theconduction band.

Conductionband

Valenceband

Hole

Freeelectron

(a) Energy diagram

Energy

Electron-hole pair

+4

+4

Hole

Freeelectron

(b) Bonding diagram

Heatenergy

Heatenergy

Band gap

FIGURE 113

Creation of electron-hole pairs in asilicon crystal. Electrons in the con-duction band are free electrons.

When an electron jumps to the conduction band, a vacancy is left in the valence bandwithin the crystal. This vacancy is called a hole. For every electron raised to the conduc-tion band by external energy, there is one hole left in the valence band, creating what iscalled an electron-hole pair. Recombination occurs when a conduction-band electronloses energy and falls back into a hole in the valence band.

To summarize, a piece of intrinsic silicon at room temperature has, at any instant, anumber of conduction-band (free) electrons that are unattached to any atom and are essen-tially drifting randomly throughout the material. There is also an equal number of holes inthe valence band created when these electrons jump into the conduction band. This is illus-trated in Figure 114.

CURRENT IN SEMICONDUCTORS 13

Electron and Hole Current

When a voltage is applied across a piece of intrinsic silicon, as shown in Figure 1-15, the ther-mally generated free electrons in the conduction band, which are free to move randomly inthe crystal structure, are now easily attracted toward the positive end. This movement of freeelectrons is one type of current in a semiconductive material and is called electron current.

Si SiSi Si Si

Si SiSi Si Si

Si SiSi Si Si

Si SiSi Si Si

Generation of anelectron-hole pair

Recombination ofan electron witha hole

Heat energy

FIGURE 114

Electron-hole pairs in a silicon crystal.Free electrons are being generatedcontinuously while some recombinewith holes.

Another type of current occurs in the valence band, where the holes created by the freeelectrons exist. Electrons remaining in the valence band are still attached to their atomsand are not free to move randomly in the crystal structure as are the free electrons.However, a valence electron can move into a nearby hole with little change in its energylevel, thus leaving another hole where it came from. Effectively the hole has moved fromone place to another in the crystal structure, as illustrated in Figure 116. Although currentin the valence band is produced by valence electrons, it is called hole current to distinguishit from electron current in the conduction band.

As you have seen, conduction in semiconductors is considered to be either the move-ment of free electrons in the conduction band or the movement of holes in the valenceband, which is actually the movement of valence electrons to nearby atoms, creating holecurrent in the opposite direction.

It is interesting to contrast the two types of charge movement in a semiconductor withthe charge movement in a metallic conductor, such as copper. Copper atoms form a differ-ent type of crystal in which the atoms are not covalently bonded to each other but consistof a sea of positive ion cores, which are atoms stripped of their valence electrons. Thevalence electrons are attracted to the positive ions, keeping the positive ions together andforming the metallic bond. The valence electrons do not belong to a given atom, but to thecrystal as a whole. Since the valence electrons in copper are free to move, the applicationof a voltage results in current. There is only one type of currentthe movement of freeelectronsbecause there are no holes in the metallic crystal structure.

Si SiSi Si

Si SiSi Si Si

Si SiSi Si Si

V

Si

+

FIGURE 115

Electron current in intrinsic silicon isproduced by the movement of ther-mally generated free electrons.


Since semiconductors are generally poor conductors, their conductivity can be drasti-cally increased by the controlled addition of impurities to the intrinsic (pure) semiconductivematerial. This process, called doping, increases the number of current carriers (electronsor holes). The two categories of impurities are n-type and p-type.

N-Type SemiconductorTo increase the number of conduction-band electrons in intrinsic silicon, pentavalent im-purity atoms are added. These are atoms with five valence electrons such as arsenic (As),phosphorus (P), bismuth (Bi), and antimony (Sb).

A free electronleaves hole invalence shell.

A valence electron movesinto 2nd hole and leavesa 3rd hole.

A valence electron movesinto 4th hole and leavesa 5th hole.

A valence electron movesinto 1st hole and leavesa 2nd hole.

A valence electron movesinto 3rd hole and leavesa 4th hole.

A valence electron movesinto 5th hole and leavesa 6th hole.

When a valence electron moves left to right to fill a hole while leaving another hole behind, the holehas effectively moved from right to left. Gray arrows indicate effective movement of a hole.

5 31

246

Si Si Si

FIGURE 116

Hole current in intrinsic silicon.

1. Are free electrons in the valence band or in the conduction band?

2. Which electrons are responsible for electron current in silicon?

3. What is a hole?

4. At what energy level does hole current occur?

SECTION 13 CHECKUP

14 N-TYPE AND P-TYPE SEMICONDUCTORSSemiconductive materials do not conduct current well and are of limited value in theirintrinsic state. This is because of the limited number of free electrons in the conductionband and holes in the valence band. Intrinsic silicon (or germanium) must be modified byincreasing the number of free electrons or holes to increase its conductivity and make ituseful in electronic devices. This is done by adding impurities to the intrinsic material.Two types of extrinsic (impure) semiconductive materials, n-type and p-type, are the keybuilding blocks for most types of electronic devices.


Describe the properties of n-type and p-type semiconductors Define doping

Explain how n-type semiconductors are formed Describe a majority carrier and minority carrier in n-type material

Explain how p-type semiconductors are formed Describe a majority carrier and minority carrier in p-type material

N-TYPE AND P-TYPE SEMICONDUCTORS 15

As illustrated in Figure 117, each pentavalent atom (antimony, in this case) forms co-valent bonds with four adjacent silicon atoms. Four of the antimony atoms valence elec-trons are used to form the covalent bonds with silicon atoms, leaving one extra electron.This extra electron becomes a conduction electron because it is not involved in bonding.Because the pentavalent atom gives up an electron, it is often called a donor atom. Thenumber of conduction electrons can be carefully controlled by the number of impurityatoms added to the silicon. A conduction electron created by this doping process does notleave a hole in the valence band because it is in excess of the number required to fill thevalence band.

Majority and Minority Carriers Since most of the current carriers are electrons, silicon(or germanium) doped with pentavalent atoms is an n-type semiconductor (the n stands forthe negative charge on an electron). The electrons are called the majority carriers inn-type material. Although the majority of current carriers in n-type material are electrons,there are also a few holes that are created when electron-hole pairs are thermally gener-ated. These holes are not produced by the addition of the pentavalent impurity atoms.Holes in an n-type material are called minority carriers.

P-Type SemiconductorTo increase the number of holes in intrinsic silicon, trivalent impurity atoms are added.These are atoms with three valence electrons such as boron (B), indium (In), and gallium(Ga). As illustrated in Figure 118, each trivalent atom (boron, in this case) forms covalentbonds with four adjacent silicon atoms. All three of the boron atoms valence electrons areused in the covalent bonds; and, since four electrons are required, a hole results when eachtrivalent atom is added. Because the trivalent atom can take an electron, it is often referredto as an acceptor atom. The number of holes can be carefully controlled by the number oftrivalent impurity atoms added to the silicon. A hole created by this doping process is notaccompanied by a conduction (free) electron.

Majority and Minority Carriers Since most of the current carriers are holes, silicon (orgermanium) doped with trivalent atoms is called a p-type semiconductor. The holes are themajority carriers in p-type material. Although the majority of current carriers in p-typematerial are holes, there are also a few conduction-band electrons that are created whenelectron-hole pairs are thermally generated. These conduction-band electrons are not pro-duced by the addition of the trivalent impurity atoms. Conduction-band electrons in p-typematerial are the minority carriers.

Free (conduction) electronfrom Sb atom

SbSi

Si

Si

Si

FIGURE 117

Pentavalent impurity atom in a sili-con crystal structure. An antimony(Sb) impurity atom is shown in thecenter. The extra electron from theSb atom becomes a free electron.


Hole from B atom

BSi

Si

Si

Si

FIGURE 118

Trivalent impurity atom in a siliconcrystal structure. A boron (B) impu-rity atom is shown in the center.

1. Define doping.

2. What is the difference between a pentavalent atom and a trivalent atom?

3. What are other names for the pentavalent and trivalent atoms?

4. How is an n-type semiconductor formed?

5. How is a p-type semiconductor formed?

6. What is the majority carrier in an n-type semiconductor?

7. What is the majority carrier in a p-type semiconductor?

8. By what process are the majority carriers produced?

9. By what process are the minority carriers produced?

10. What is the difference between intrinsic and extrinsic semiconductors?

SECTION 14 CHECKUP

15 THE PN JUNCTIONWhen you take a block of silicon and dope part of it with a trivalent impurity and the otherpart with a pentavalent impurity, a boundary called the pn junction is formed between theresulting p-type and n-type portions. The pn junction is the basis for diodes, certain transis-tors, solar cells, and other devices, as you will learn later.


Describe how a pn junction is formed Discuss diffusion across a pn junction

Explain the formation of the depletion region Define barrier potential and discuss its significance State the values of barrierpotential in silicon and germanium

Discuss energy diagrams Define energy hill

A p-type material consists of silicon atoms and trivalent impurity atoms such as boron.The boron atom adds a hole when it bonds with the silicon atoms. However, since the num-ber of protons and the number of electrons are equal throughout the material, there is nonet charge in the material and so it is neutral.

THE PN JUNCTION 17

An n-type silicon material consists of silicon atoms and pentavalent impurity atoms such asantimony. As you have seen, an impurity atom releases an electron when it bonds with foursilicon atoms. Since there is still an equal number of protons and electrons (including the freeelectrons) throughout the material, there is no net charge in the material and so it is neutral.

If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type,a pn junction forms at the boundary between the two regions and a diode is created, asindicated in Figure 119(a). The p region has many holes (majority carriers) from theimpurity atoms and only a few thermally generated free electrons (minority carriers). Then region has many free electrons (majority carriers) from the impurity atoms and only afew thermally generated holes (minority carriers).

p region n region

pn junction

(a) The basic silicon structure at the instant of junction formationshowing only the majority and minority carriers. Free electronsin the n region near the pn junction begin to diffuse across thejunction and fall into holes near the junction in the p region.

p region n region

Depletion region

+

+

+

+

+

+

+

+

Barrierpotential

For every electron that diffuses across the junction andcombines with a hole, a positive charge is left in the n regionand a negative charge is created in the p region, forming abarrier potential. This action continues until the voltage ofthe barrier repels further diffusion. The blue arrows betweenthe positive and negative charges in the depletion regionrepresent the electric field.

(b)

FIGURE 119

Formation of the depletion region. The width of the depletion region is exaggerated for illustrationpurposes.

After the invention of the light bulb,Edison continued to experiment andin 1883 found that he could detectelectrons flowing through thevacuum from the lighted filament toa metal plate mounted inside thebulb. This discovery became knownas the Edison effect.

An English physicist, JohnFleming, took up where Edison leftoff and found that the Edison effectcould also be used to detect radiowaves and convert them to electricalsignals. He went on to develop atwo-element vacuum tube called theFleming valve, later known as thediode. Modern pn junction devicesare an outgrowth of this.

H I S T O R Y N O T EFormation of the Depletion Region

The free electrons in the n region are randomly drifting in all directions. At the instant ofthe pn junction formation, the free electrons near the junction in the n region begin to dif-fuse across the junction into the p region where they combine with holes near the junction,as shown in Figure 119(b).

Before the pn junction is formed, recall that there are as many electrons as protons inthe n-type material, making the material neutral in terms of net charge. The same is true forthe p-type material.

When the pn junction is formed, the n region loses free electrons as they diffuse acrossthe junction. This creates a layer of positive charges (pentavalent ions) near the junction.As the electrons move across the junction, the p region loses holes as the electrons andholes combine. This creates a layer of negative charges (trivalent ions) near the junction.These two layers of positive and negative charges form the depletion region, as shown inFigure 119(b). The term depletion refers to the fact that the region near the pn junction isdepleted of charge carriers (electrons and holes) due to diffusion across the junction. Keepin mind that the depletion region is formed very quickly and is very thin compared to the nregion and p region.

After the initial surge of free electrons across the pn junction, the depletion region hasexpanded to a point where equilibrium is established and there is no further diffusion of


electrons across the junction. This occurs as follows. As electrons continue to diffuseacross the junction, more and more positive and negative charges are created near the junc-tion as the depletion region is formed. A point is reached where the total negative charge inthe depletion region repels any further diffusion of electrons (negatively charged particles)into the p region (like charges repel) and the diffusion stops. In other words, the depletionregion acts as a barrier to the further movement of electrons across the junction.

Barrier Potential Any time there is a positive charge and a negative charge near eachother, there is a force acting on the charges as described by Coulombs law. In the depletion re-gion there are many positive charges and many negative charges on opposite sides of the pnjunction. The forces between the opposite charges form an electric field, as illustrated inFigure 119(b) by the blue arrows between the positive charges and the negative charges. Thiselectric field is a barrier to the free electrons in the n region, and energy must be expended tomove an electron through the electric field. That is, external energy must be applied to get theelectrons to move across the barrier of the electric field in the depletion region.

The potential difference of the electric field across the depletion region is the amount ofvoltage required to move electrons through the electric field. This potential difference iscalled the barrier potential and is expressed in volts. Stated another way, a certainamount of voltage equal to the barrier potential and with the proper polarity must be ap-plied across a pn junction before electrons will begin to flow across the junction. You willlearn more about this when we discuss biasing in Chapter 2.

The barrier potential of a pn junction depends on several factors, including the type ofsemiconductive material, the amount of doping, and the temperature. The typical barrierpotential is approximately 0.7 V for silicon and 0.3 V for germanium at Because ger-manium devices are not widely used, silicon will be used throughout the rest of the book.

Energy Diagrams of the PN Junction and Depletion RegionThe valence and conduction bands in an n-type material are at slightly lower energy levelsthan the valence and conduction bands in a p-type material. Recall that p-type material hastrivalent impurities and n-type material has pentavalent impurities. The trivalent impuritiesexert lower forces on the outer-shell electrons than the pentavalent impurities. The lowerforces in p-type materials mean that the electron orbits are slightly larger and hence havegreater energy than the electron orbits in the n-type materials.

An energy diagram for a pn junction at the instant of formation is shown in Figure120(a). As you can see, the valence and conduction bands in the n region are at lower en-ergy levels than those in the p region, but there is a significant amount of overlapping.

The free electrons in the n region that occupy the upper part of the conduction band interms of their energy can easily diffuse across the junction (they do not have to gain addi-tional energy) and temporarily become free electrons in the lower part of the p-region con-duction band. After crossing the junction, the electrons quickly lose energy and fall intothe holes in the p-region valence band as indicated in Figure 1-20(a).

As the diffusion continues, the depletion region begins to form and the energy level ofthe n-region conduction band decreases. The decrease in the energy level of the conductionband in the n region is due to the loss of the higher-energy electrons that have diffusedacross the junction to the p region. Soon, there are no electrons left in the n-region conduc-tion band with enough energy to get across the junction to the p-region conduction band, asindicated by the alignment of the top of the n-region conduction band and the bottom of thep-region conduction band in Figure 120(b). At this point, the junction is at equilibrium;and the depletion region is complete because diffusion has ceased. There is an energy gra-diant across the depletion region which acts as an energy hill that an n-region electronmust climb to get to the p region.

Notice that as the energy level of the n-region conduction band has shifted downward,the energy level of the valence band has also shifted downward. It still takes the sameamount of energy for a valence electron to become a free electron. In other words, the en-ergy gap between the valence band and the conduction band remains the same.

25C.

Russell Ohl, working at Bell Labsin 1940, stumbled on thesemiconductor pn junction. Ohlwas working with a silicon samplethat had an accidental crack downits middle. He was using anohmmeter to test the electricalresistance of the sample when henoted that when the sample wasexposed to light, the current thatflowed between the two sides ofthe crack made a significant jump.This discovery was fundamental tothe work of the team that inventedthe transistor in 1947.

H I S T O R Y N O T E

SUMMARY 19

1. What is a pn junction?

2. Explain diffusion.

3. Describe the depletion region.

4. Explain what the barrier potential is and how it is created.

5. What is the typical value of the barrier potential for a silicon diode?

6. What is the typical value of the barrier potential for a germanium diode?

SECTION 15 CHECKUP

Majority carriersMinority carr

Electronic devices-9th-edition-by-floyd

Engineering

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