Electronic Devices and Circuits

(1)

second

edition

(2)

(3)

(4)

(5)

Electronic

Devices

(6)

(7)

Electronic

Devices

and

Circuits

2nd

Edition

David

A. Bell

Lambton

College

of

Applied

Arts

and

Technology

Sarnia,

Ontario,

Canada

n

I

Reston

Publishing

Company,

Inc., Reston, Virginia

(8)

LibraryofCongressCataloging in PublicationData

Bell,DavidA.

Electronicdevicesandcircuits.

Includesindex.

1. Semiconductors. 2. Electroniccircuits. 3. Electronic apparatusandappliances. I. Title.

TK7871.85.B3785 1980 621.3815 79-22957

ISBN

0-8359-1634-0

©

1980by

RestonPuUishing

Company,

Inc.

A

Prentice-HallCompany

Reston, Virginia22090

No

partof thisbook

may

be reproducedinanyway, orbyanymeans,v»athout

permissioninwritingfromthepublisher.

10

98765432

PrintedintheUnitedStates ofAmerica

(9)

(10)

(11)

Chapter

1

BASIC

SEMICONDUCTOR THEORY

1

1-1 Introduction 1 1-2

The

Atom

1

1-3 ElectronOrbits

and Energy

Levels 3

1-4

Energy Bands

4

1-5

Conduction

in Solids 5

1-6 Conventional Current

and

Electron

Flow

6

1-7

Bonding

Forces

Between

Atoms

7

1-8 Conductors,Insulators,

and

Semiconductors 8

1-9 Semiconductor

Doping

9

1-10 Effects of

Heat

and

Light 1 1

1-11 DriftCurrent

and

DiffusionCurrent 12 Glossary of ImportantTerms 13

ReviewQuestions 15

(12)

viii

Chapter

2 pn-JUNCTION

THEORY

16

Contents 2-1 Introduction 16

2-2

The

/OT-Junction 16

2-3 Reverse Biased Junction 19

2-4

Forward

Biased Junction 2

1

2-5

Temperature

Effects 23

2-6 Junction Capacitance 25

2-7 Junction EquivalentCircuit 25

Glossaryof ImportantTerms 26 ReviewQuestions 27

Chapter

3 The

Semiconductor

Diode

29

3-1 Introduction 29

3-2

Diode

Symbol

and Appearance

29

3-3

Diode

Fabrication 31

3-4

Diode

Characteristics

and

Parameters 32

3-5 Graphical Analysisof

Diode

Circuit 33

3-6

Diode

PiecewiseLinearCharacteristics 38

3-7

Diode

EquivalentCircuit 39

3-8

Diode

Data

Sheet 40

3-9

Half-Wave

Rectification 43

3-10

Full-Wave

Rectification 49

3-11

Diode

Switching

Time

and Frequency

Response 53

3-12

Diode

LogicCircuits 55

3-13

Diode

ClipperCircuits 56

3-14 Voltage MultiplierCircuit 58

Glossaryof ImportantTerms 60 ReviewQuestions 61 Problems 62

Chapter 4 The

Junction Transistor 65

4-1 Introduction 65

4-2 TransistorOperation 65

4-3 TransistorCurrents 70

4-4 Transistor

Symbols

and

Voltages 73

4-5

Common

BaseCharacteristics 74

4-6

Common

EmitterCharacteristics 78

4-7

Common

Collector Characteristics 81

4-8 TransistorT-EquivalentCircuit

and

r-Parameters 83

4-9 A-Parameters 84

Glossaryof Important Terms 89 ReviewQuestions 90

(13)

Chapter

5

Transistor Biasing 93 ix

5-1 Introduction 93 Contents

5-2

The

dcLxjad Line

and

BiasPoint 94

5-3 FixedCurrentBias 98 5-4 CoUector-to-BaseBias 100

5-5 EmitterCurrentBias (or Self Bias) 102

5-6

Comparison

ofBasic Bias Circuits 107

5-7

Thermal

Stability 107

5-8 ac Bypassing

and

theac

Load

Line 110

Glossary of ImportantTerms 113 ReviewQuestions 114

Problems 114

Chapter 6

Basic TransistorCircuits 116

6-1 Introduction 116

6-2

Common

EmitterCircuit 116

6-3

Common

EmitterA-parameter Analysis 118 6-4

Common

Collector Circuit 125

6-5

Common

CollectorA-Parameter Analysis 126 6-6

Common

BaseCircuit 131

6-7

Common

BaseA-Parameter Analysis 132

6-8

Cascaded

Common

EmitterCircuits 139

Glossary of ImportantTerms 141

ReviewQuestions 141 Problems 142

Chapter

7

Transistor

and

IntegratedCircuit Fabrication

143

7-2 Effects ofTransistorConstruction

on

Electrical

Performance

143

7-3 ProcessingofSemiconductor Materials 144

7-4 TransistorFabrication 146

7-5 IntegratedCircuitFabrication 150

7-6 Integrated Circuit

Components

152

7-7 Transistor

and

Integrated CircuitPackaging 154

Chapter

8

Transistor Specifications

and

Performance

158

(14)

8-3

Power

Dissipation 163

8-4 Decibels

and Frequency

Response 165

8-5 MillerEffect 170

8-6 Transistor CircuitNoise 171 8-7 TransistorSwitching 175

Problems 180

Chapter

9

Basic Multistage

and

IntegratedCircuitAmplifiers

182

9-2 Capacitor-Coupled

Two-Stage

Circuit 183 9-3 Direct

Coupled Two-Stage

Circuit 188 9-4

The

DifferentialAmplifier 192 9-5

IC

DifferentialAmplifiers 200

9-6 Basic

IC

OjierationalAmplifierCircuits

204

9-7 Transformer

Coupled

Class

A

Amplifier 211

9-8 Transformer

Coupled

Class

B

and

Class

AB

Circuits 216 9-9 Multistage Emitter Followers 222

Problems 228

Chapter

10 BasicSinusoidalOscillators

230

10-2 Phase-Shift Oscillator 230

10-3 ColpittsOscillator 234

10-4 HartleyOscillator 237

10-5

Wein

BridgeOscillator 240

Problems 244

Chapter

11

Zener Diodes

245

11-2

Zener

and Avalanche

Breakdown

245

11-3

Zener Diode

Characteristic

and

Parameters 247

11-4 Comf)ensated Reference Diodes 251

11-5

Zener Diode

Voltage Regulator 252

11-6 Regulator

With

Reference

Diode

257

11-7

Other Zener Diode

Applications 257

(15)

Chapter

12 Field EffectTransistors 262 xi

12-1 Introduction 262 Contents 12-2 Principle of then-Channel

JFET

262

12-3 Characteristics ofn-Channel

JFET

264

12-4

The

/(-Channel

JFET

268

12-5

JFET

Data

Sheet

and

Parameters 269

12-6

JFET

Construction 276

12-7

FET

EquivalentCircuit 278

12-8

The

MOSFET

278

12-9

The

V-MOSFET

282

Problems 287

Chapter

13

FET

Biasing 289

13-2 dc

Load

Line

and

BiasPoint 289

13-3 SpreadofCharacteristics

and

FixedBias Circuit 291

13-4 Self-bias 293

13-5 Self-biaswith ExternalVoltage 296

13-6 Designof

FET

Bias Circuits 298

13-7 Biasing

MOSFETS

300

Glossaryof Important Terms 303

Problems 303

Chapter

14 Basic

FET

Circuits 308

14-2

The

Common

SourceCircuit 308

14-3 ac Analysisof

Common

SourceCircuit 310

14-4

The

Common

Drain

Circuit 313

14-5 ac Analysisof

Common

DrainCircuit 315

14-6

The

Common

Gate

Circuit 318

14-7 ac Analysisofthe

Common

Gate

Circuit

319

14-8

BI-FET

and

BI-MOS

Circuits 322

Problems 325

Chapter

15

The

Tunnel

Diode

327

15-2

Theory

ofOperation 327

(16)

(17)

18-11

Gas

Discharge Displays

404

xlii

18-12 OptoelectronicCouplers 405 Contents 18-13 Laser

Diode

407

Problems 411

Chapter

19

Miscellaneous

Devices

414

19-1 Piezoelectricity 414

19-2 PiezoelectricCrystals 414

19-3 SyntheticPiezoelectricDevices 421

19-4 Voltage-Variable Capacitor Diodes 422

19-5 Thermistors 427

19-6

Lambda

Diode

432

Problems 435

Chapter

20

Electron

Tubes

437

20-1 Introduction 437 20-2

The

Vacuum

Diode

438 20-3

The

Vacuum

Triode 441 20-4 TriodeCharacteristics 442 20-5 Triode Parameters 445 20-6

Common

Cathode

Circuit 447

20-7 ac Analysisof

Common

Cathode

Circuit

449

20-8

Common

Plate Circuit 453 20-9

Common

GridCircuit 454 20-10 Triode Biasing

Methods

454

20-11

The

Tetrode

Tube

457 20-12

The

Pentode 460

20-13

The

Variable-Mu

or

Remote

CutoffPentode 462 20-14

The

Cathode

Ray Tube

463

Problems 475

Appendix

1 Typical

Standard

ResistorValues

477 Appendix

2

Typical

Standard

Capacitor

Values

478 Answers

to

Problems

479

(18)

(19)

Preface

This is the second edition of Fundamentals ofElectronic Devices,

now

renamed

ElectronicDevicesandCircuitsto

more

correcdydescribe thecontents

ofthe book.Asinthefirstedition,

my

objectivesaretoclearlyexplainthe

operationofallimportantelectronicdevicesingeneralusetoday

and

togive thereader athorough understandingofthe characteristics,parameters,

and

circuit applications ofeachdevice. In addition, I attemptto

show

a basic

approach

todesigningeach deviceinto practicalcircuits.

The

bookisintendedforuseinelectronicstechnologycourses,whether

two-, three-,orfour-year courses. Itshouldalso proveuseful asareference

handbook

forpracticing technicians, technologists,

and

engineers.

The

text

commences

withthestudyofbasicsemiconductor theory

and

/w-junctiontheory

which

isessentialfor

an

understandingofallsolid-state devices.

Each

differentdevice isthen treated inappropriate depth, begin-ning,of course, with thesemiconductordiode,then the bipolartransistor.

Transistor bias circuits, single-stage amplifiers, multistage amplifiers,

and

(20)

xvi integratedcircuitapplicationsarecombined.

The

integrated circuit

ojjera-Preface

tionalamplifier

and

itsbasicapplications areexplainedinthe chapters

on

multistage amplifiers

and

oscillators.

Althoughuseful

background

informationforeach deviceisincludedin thebook, everyefforthasbeen

made

toeliminateunnecessarymaterial.For example,transistor

and

integrated circuit fabricationtechniquesarecovered onlyfromthepoint ofviewof

how

deviceperformanceisaffected.

As

well as bipolar transistors

and

integrated circuits, other devices

coveredinclude:

Zener

diode,

JFET,

MOSFET,

VMOS

FET,

tunneldiode,

SCR, UJT, PUT,

photoconductive cell, solar cell, phototransistor,

LED,

LCD,

piezoelectriccrystal,

WC

diode,

and

thermistor.Sinceelectron tubes arestillinwideuseinexistingequipment,thefinalchaptercoversitsvaried

forms:

vacuum

diode, triode, tetrode, pentode, and, of course, the very important cathode-raytube.

Throughout

the

book

many

examplesare

employed

toexplain

practi-calapplications ofeachdevice.Insteadofrigorous analysismethods, practi-cal approximations are used wherever possible,

and

the origin of each approximation is explained. Manufacturers' data sheets are referred to

where

appropriate.Problems are providedateach chapter end,

and

answers

to allproblemsarefound inthe backofthebook.Glossariesofimpwrtant termsare alsoincludedatthe

end

ofeachchapter.

The

mathematicslevelthroughout the textdoes notgo

beyond

alge-braicequations

and

logarithms,simply because

no

higher

math

isnecessary

to fulfill the purpose of the book. It is expected that studentswill have alreadystudiedbjisicelectricity.

(21)

Basic

Semiconductor

Tiieory

CHAPTER

1 The

functionof

an

electronicdeviceistocontrol the

movement

ofelectrons.

The

firststep in a studyof such devicesis tounderstand the.electron (or

what

itisbelievedto be),

and

how

itisassociatedwiththeother

components

of the atom. After such

an

understanding is reached the

bonding

forces

holding

atoms

togetherwithin asolid

and

the

movement

ofelectronsfrom one

atom

toanothermust beinvestigated.Thisleadsto

an

understandingof the differencesbetweenconductors, insulators,

and

semiconductors.

1-1 Introduction

The

atom

is believed to consist of a central nucleus surrounded by

orbitingelectrons(see Fig. 1-1). Thus,it

may

be

compared

toaplanet with

satellitesinorbit

around

it.Just assatellitesareheldinorbitby

an

attractive forceofgravity

due

tothe

mass

ofthe planet, soeachelectronisheldinorbit

by

an

electrostaticforceofattractionbetweenit

and

the nucleus.

The

electronseach have anegative electricalchargeof 1.602X10

~"

coulombs(C),

and some

particleswithinthenucleushave a{xwitivechargeof the

same

magnitude. Since opposite charges attract, a force of attraction

1-2

(22)

2

Basic

Semiconductor Theory

'_Nucleus

(a)Nucleuswith orbiting electrons

(b)Forcesonsatellite orbiting aplanet

(c)Forcesonelectrons orbitinganucleus

Figure1-1. Planetaryatom.

exists

between

the oppositelychargedelectron

and

nucleus.

As

inthe case of thesatellites,the force of attractionisbalanced

by

the centrifugal force

due

to themotionofthe electrons

around

thenucleus[Fig. l-l(b)

and

(c)].

Compared

to the mass of the nucleus, electrons are relatively tiny particles ofalmostnegligiblemass.Infact,

we

may

thinkof

them

simplyas litdeparticlesofnegativeelectricityhaving no

mass

atall.

The

nucleusof

an

atom

islargely aclusterof

two

types ofparticles, protons

and

neutrons. Protons have a posidve electrical charge, equal in

magnitude

(but opposite in polarity)tothenegativecharge

on

an

electron.

A

neutron has

no

charge atall. Protons

and

neutronseach have masses about 1800

dmes

the mjiss of

an

electron.For a given atom, the

number

of

protonsinthenucleusnormally equalsthe

number

oforbidngelectrons.

Sincetheprotons

and

orbital electrons areequalin

number and

equal

and

opp)osite in charge, they neutralize each other electrically. For this

reason, all atoms are normally electrically neutral. If

an

atom

loses

an

electron, ithas lost

some

negative charge. Therefore,it

becomes

fxjsiuvely

charged

and

isreferred to as apositiveion. Similarly, if

an

atom

gains

an

additional electron,it

becomes

negativelycharged

and

istermed anegativeion.

The

differences between atoms consist largely ofdissimilar

numbers

and

arrangementsofthe three basic types of pardcles.

However,

allelectrons

(23)

are identical, asareallprotons

and

allneutrons.

An

electronfrom one

atom

couldreplace

an

electron in

any

otheratom. Different materials are

made

up

ofdifferenttypes ofatoms, ordifferingcombinationsofseveral types of

atoms.

The number

ofprotons(or electrons)in

an

atom

isreferred to as the atomic numberof the atom.

The

atomic weightisapproximately equal tothe

total

number

ofprotons

and

neutronsinthenucleusoftheatom.

The

atom

ofthesemiconductor element siliconhas 14 protons

and

14 neutronsin its

nucleus, as well as 14 orbital electrons. Therefore, theatomic

number

for

siliconis 14,

and

itsatomic weightisapproximately28.

3

Electron Orbitsand EnergyLevels

Atoms

may

be conveniently represented

by

thetwo-dimensional

dia-grams

shown

in Fig. 1-2. Ithasbeenfound that electronscan

occupy

only

certain orbital ringsor shellsat fixeddistancesfromthe nucleus,

and

that

eachshell

can

containonlyaparticular

number

of electrons.

The

electrons intheoutershelldeterminetheelectrical (and chemical)characteristics of

each particular type ofatom. These electrons are usually referred to as

valenceelectrons.

An

atom

may

haveitsouterorvalence shellcompletelyfilled

oronlypartiallyfilled.

The

atoms

oftwo important semiconductors,silicon(Si)

and

germanium

(Ge),are illustrated inFig. 1-2. Itisseen thateachofthese

atoms

havefour electronsina valenceshellthatcan contain a

maximum

ofeight.Thus,

we

saythat their valenceshells have four electrons

and

fourholes.

A

hole is

definedsimplyas

an

absenceof

an

electroninashell

where

one couldexist.

Even

though their valence shells have four holes, both silicon

and

germanium atoms

arestillelectricallyneutral,becausethetotal

number

of

orbitalelectronsequalsthetotal

number

ofprotonsinthe nucleus.

1-3

Electron Orbits

and

EnergyLevels

^-0—0-.

e,0'

e.0

/

-©'~^~0.

^Q

III/

Nucleus

cj^

i'

4 o

A

^cr

/ , / Nucleus ^ \ \

4 O

Ki

t

(b)Siliconatom (a)Germaniumatom

(24)

4

Basic

The

closer

an

electronisto the nucleus, the stronger are the forces that

bindit.

Each

shellhas

an

energylevelassociatedwithit

which

represents the

amount

ofenergythat

would

havetobe suppliedtoextract

an

electronfrom

the shell. Since the electrons in the valence shell are farthest from the nucleus,they require theleast

amount

ofenergyto extract

them

fromthe

atom. Conversely,those electronsclosesttothenucleusrequire the greatest

energyapplication to extract

them

fromtheatom.

The

energylevelsconsideredabove are

measured

inelectronvolts(eV).

An

electron voltis defined asthe

amount

ofenergy required to

move

one

electronthrough apotentijildifference ofonevolt.

1-4

Sofarthe discussionhasconcerned a systemofelectrons

around

one Energy

Bands

isolatedatom.

The

electrons of

an

isolated

atom

areactedup)ononlybythe forceswithinthatatom.

However,

when

atomsarebroughtclosertogether as inasolid,the electrons

come

under

the influenceofforcesfrom other atoms.

The

energylevelsthat

may

be occupied byelectrons

merge

intobandsof

energylevels.

Within

any

given materialtherearetwodistinctenergybands in

which

electrons

may

exist,thevalenceband

and

theconductionband.Separating

thesetwo

bands

is

an

energygap in

which no

electrons

can

normallyexist.

This

gap

is termed the forbidden gap.

The

valenceband, conductionband,

and

forbidden

gap

are

shown

diagrammaticallyinFig. 1-3.

Electronswithintheconduction

band

have

become

disconnectedfrom atoms

and

are drifting

around

within the material.

Conduction

band

electrons

may

beeasily

moved

around

by

the applicationofrelativelysmall

amounts

of energy.

Much

larger

amounts

of energy

must

be applied to

extract

an

electronfrom thevalence

band

orto

move

it

around

vsdthinthe

valenceband. Electrons in the valence

band

are usually in

normal

orbit

around

anucleus.For

any

given typeofmaterial,theforbidden

gap

may

be

large,small,or nonexistent.

The

distinctionbetweenconductors, insulators,

and

semiconductors is largely concerned with the relative widths of the

forbidden gap.

It is important to note that the energy

band diagram

is simply a graphic representation of the energy levels associated with electrons.

To

Energy

level

Conduction band

Forbidden gap

Valence band

(25)

repeat, those electrons in thevalence

band

are actually in orbit

around

the

nucleusof

an

atom; thosein theconduction

band

are driftingaboutin the spacesbetween atoms.

Conduction

inSolids

Conduction

occurs in

any

given material

when

an

applied voltage

causes electronswithinthematerialto

move

inadesired direction.This

may

be

due

tooneor both of two processes, electron motion

and

hole transfer. In electronmotion,free electrons inthe conduction

band

are

moved

under the influence of theappliedelectric field.Sinceelectronshave a negativecharge,

they are repelled from the negative terminal of the applied voltage,

and

attracted toward the positive terminal.

Hole

transfer involves electrons

which

arestillattachedtoatoms,i.e.,thoseinthevalence band.

If

some

oftheenergylevelsin thevalence

band

are notoccupiedby

electrons,thereare holes

where

electronscouldexist.

An

electron

may jump

fromone

atom

tofilltheholeinanotheratom.

When

itjumps,the electron leavesaholebehindit,

and

we

say that the hole has

moved

inthe opposite directiontothe electron.Inthis

way

a currentflows

which

may

besaidtobe

due

tohole

movement.

InFig. l-4(a), theapplied potentialcauses

an

electronto

jump

from

atom^

to

atom

x.Indoingso,itfillsthehole in thevalenceshellof

atom

x,

and

leavesaholebehinditin

atom^

as

shown

inFig. l-4(b).If

an

electron

now

jumps

from

atom

z, underthe influence oftheappliedpotential,

and

fills the hole in the valenceshell of

atom

y, it leaves a hole in

atom

z

[Fig. l-4(c)].Thus,theholehasbeen causedto

move

from

atom

xtoatom^y

to

atom

z.

Holes

may

be thoughtofas positive particles,

and

assuchthey

move

throughanelectric field inadirection opjxwite to that of the electrons; i.e..

1-5

Conduction

inSolids (a) (b)

X

(26)

6

Basic

positive particles are attracted toward the negative terminalof

an

applied

voltage. Itis usually

more

convenient tothinkintermsofhole

movement,

ratherthanintermsof electrons

jumping

from

atom

toatom.

Sincethe flow ofelectriccurrent isconstituted

by

the

movement

of electrons in theconduction

band and

holesin thevalenceband,electrons

and

holes are referred to as charge carriers.

Each

time a hole moves,

an

electron

must

be supplied withsufficientenergytoenableittoescapefrom

itsatom. Freeelectrons requirelessapplication ofenergythanholesto

move

them, because they are already disconnected from theiratoms. For this

reason, electronshavegreater mobilitythanholes.

The

unit of electric current is the ampere (A).

An

ampere

may

be defined asthatcurrent

which

flov*^

when

one

coulomb

of chargepassesa given pointin

one

second.

From

thisdefinition

we

cancalculatethe

number

ofelectronsinvolvedina currentofone ampere. Sincethecharge

on one

electronis 1.602

X

10~'^ C,the

number

ofelectronswith atotalcharge of

1

C

is

1/(1.602X10"'®)«6.25X10'^.

When

one

microampere

(juA) flows

(i.e.,1

X

10~^

A), electronsarepassing at the rate of 6.25

X

lO'^persecond,

or1/nA

=

6,250,000,000,000electronspersecond.

1-6

Conventional

Current

and

Electron

Flow

Inthe earlydaysofelectricalexf>erimentationit

was

believed that a

positivecharge represented

an

increased

amount

ofelectricity

and

that a negativecharge

was

areducedquantity.Thus,it

was assumed

thatcurrent flowed

from

positive to negative.Thisis a conventionthatremains in use

todayeven

though

currentis

now known

tobe a

movement

ofelectronsfrom negativetopositive(see Fig. 1-5).

Conventional current direction

-Electron

motion-(^*

©^.

0r"

m

Figure1-5. Conventionalcurrent directionisfronnpositive to negative. Electronflowis fromnegative to positive.

(27)

Currentflowfrom positive to negative is referred to as the conventional

direction of current. Electron flow from negative to positive is known as the

directionofelectron flow.

Itisimpwrtanttounderstand both conventional currentdirection

and

electron flow. Every graphicsymbol usedto represent

an

electronicdevice

has

an arrowhead which

indicatesconventional current direction.

A

con-sequenceofthisisthat electroniccircuitsaremosteasilyexplainedby using current flow from positivetonegative.

However,

to understand

how

each deviceoperates,itisnecessarytothinkin termsofelectron

movement.

BondingForces

BetweenAtoms

Whether

a material isa conductor, a semiconductor,or

an

insulator

dependslargely

upon what

hapjjens to the outer-shell electrons

when

the

atoms

bond

themselves togethertoform asolid. In the case ofcopper,the easilydetached valenceelectronsaregiven

up

bytheatoms.Thiscreatesa

greatmassoffreeelectrons (orelectrongas) driftingabout throughthespaces betweenthecopper atoms. Since each

atom

haslosta(negative) electron,it

becomes

apositive ion.

The

electron gas is, ofcourse, negativelycharged;

consequently,anelectrostaticforceofattraction existsbetweenthe positive ions

and

the electron gas.This is the bondingforce thatholds the material

togetherinasolid.In the case ofcopper

and

othermetals,thelx>nding force

is termed metallic bonding or sometimes electron gas bonding. This tyjse of

bonding

isillustratedin Fig. l-6(a).

In the case ofsilicon,

which

hasfour outer-shell electrons

and

four

holes,the

bonding

arrangementisalittle

more

complicatedthanforcopf>er.

Atoms

inasolidpiece ofsiliconare so closetoeach otherthatthe outer-shell

electronsbehaveasiftheywereorbiting in thevalenceshellsoftwo atoms.

In this

way

eachvalence-shellelectronfillsoneofthe holesin thevalence

shell ofa neighboring atom. This arrangement, illustrated in Fig. l-6(b),

forms a

bonding

force

known

ascovalent bonding. Incovalent

bonding

every valenceshellofevery

atom

appearstobe filled,

and

consequently thereare

no

holes

and

nofreeelectrons drifitingabout withinthe material.

The

same

istruefor

germanium

atoms.

When

semiconductor materialispreparedfor

device manufacture, the

atoms

within the material are aligned into a

definite three-dimensional patternorcrystallattice.

Each atom

iscovalently

bonded

tothe foursurrounding atoms.

In

some

insulating materials,notablyrubber

and

plastics,the

bonding

process is also covalent.

The

valence electrons in these bonds are very

stronglyattachedtotheiratoms,sothejjossibilityofcurrentflowisvirtually zero. In other typ)es ofinsulating materials,

some atoms

have parted with

outer-shell electrons, but thesehave been accepted into the orbit ofother atoms. Thus, the atoms are ionized; those

which

gave

up

electrons have

become

positive ions,

and

those

which

accepted the electrons

become

negative

ions.Thiscreates

an

electrostatictxinding forcebetweentheatoms,termed

ionicbonding.

The

situationisillustrated in Fig. l-6(c),

which

shows

how

the

negative

and

positive ions

may

be arrangedtogether in groups.

1-7

Bonding

Forces

Between

Atoms

(28)

•" Free electrons

O

wO ^o

oj

_oOo

0[jO\},o

(b)Covalcnt bonding Sharedvalence electrons

(a)Metallicbonding

Negative ion

(c)Ionicbonding

Figure1-6. Atomic bondinginconductors, semiconductors, andinsulators.

1-8

Conductors,

Insulators,

and

Semi-conductors

As

seenintheenergy

band

diagramsof Fig. 1-7, insulatorshave a wide forbidden gap, semiconductors

have

a

narrow

forbidden gap,

and

conductors

have

no

forbidden

gap

atall. Inthe caseofinsulators,thereare practically

no

electrons intheconduction

band

ofenergylevels,

and

thevalence

band

is filled. Also,theforbidden

gap

issowide[Fig. l-7(a)] thatit

would

require

theapplication ofverylarge

amounts

ofenergy (approximately 6

eV)

to

cause

an

electron to cross fromthevalence

band

tothe conductionband. Therefore,

when

a voltage is applied to

an

insulator, conduction cannot occureither

by

electronmotionorholetransfer.

For semiconductorsatatemperatureofabsolutezero(

—

273.15°C)the

valence

band

isusuallyfull,

and

there

may

benoelectronsintheconduction band.

However,

as

shown

inFig.l-7(b), thesemiconductor forbidden

gap

is

very

much

narrower thanthat of

an

insulator,

and

the applicationofsmall

amounts

ofenergy(1.2

eV

for silicon

and

0.785

eV

for

germanium)

canraise

electrons

from

thevalence

band

totheconduction band.Sufficientthermal 8

(29)

Conduction band

Semiconductor

Doping

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

Figure1-7. Energy band diagramsfor insulator,semiconductor,and conductor.

energyfor thispurposeis

made

available

when

thesemiconductorisat

room

temperature. If a potential is applied to the semiconductor, conduction occurs both by electron

movement

in the conduction

band

and

by hole transferinthevalence band.

In the case ofconductors [Fig. l-7(c)]there is

no

forbiddengap,

and

thevalence

and

conductionenergybandsoverlap.Forthisreason,very large

numbers

ofelectronsare available for conduction, even at extremely

lovk-temperatures.

Typicalresistance valuesfora1-cubic-centimetersampleare

Conductor

Semiconductor Insulator 10"^

n/cm^

10fi/cm' 10'*

n/cm^

Pure semiconductor materialisreferredtoasintrinsicmaterial. Before semiconductormaterialcan be usedfordevicemanufacture, impurity

atoms

must be

added

to it. This process is called doping,

and

it improves the conductivity of the material very significantly. Dojjed semiconductor

material is termed extrinsic material. Tv^fo different types of doping arc possible, donor doping

and

acceptor doping. Donor doping generates free

electrons in the conduction

band

(i.e., electrons that are not tied to

an

atom).Acceptordoping produces valence

band

holes,ora shortageofvalence

electronsinthe material.

Donor doping is effected

by

adding impurity

atoms which

have five

electrons

and

three holes in theirvalenceshells.

The

impurity atoms form covalentbonds withthesiliconor

germanium

atoms; butsince

semiconduc-tor

atoms

have onlyfour electrons

and

four holesintheirvalenceshells,one

1-9

Semi-conductor

(30)

10

Basic

Fifthvalence electron

fromimpurityatom becomesfreeelectron

Impurityatom

Figure1-8. Donordoping.

sparevalence-shell electronisproducedforeach impurity

atom

added.

Each

spare electron produced in this

way

enterstheconduction

band

asa free

electron.InFig.1-8 thereis

no

holeforthefifthelectron

from

theoutershell

ofthe impurityatom; therefore, it

becomes

afreeelectron. Sincethe free electrons

have

negativecharges, donor-doped material is

known

asn-type

sfemiconductormaterial.

Freeelectronsintheconduction

band

are easily

moved

around under

the influence of

an

electric field. Therefore, conduction occurs largely

by

electron motion in donor-doped semiconductor material.

The

doped

material remainselectricallyneutral (i.e.,itisneither positively nor

nega-tivelycharged), becausethe total

number

ofelectrons (includingthe free

electrons) isstillequaltothetotal

number

ofprotonsintheatomicnuclei.

(The

number

ofprotonsineach impurity

atom

isequziltothe

number

of orbital electrons.)

The

termdonor doping

comes from

thefactthat

an

electron

is donatedto theconduction

band

by each impurity atom. Typical donor

impurities areantimony, phosphorus,

and

arsenic. Sincethese

atoms

havefive

valenceelectrons,theyare referredtoaspentavalentatoms.

Inacceptordoping,impurityatoms are

added

with outershells

contain-ingthree electrons

and

fiveholes. Suitableatoms with threevalence elec-trons(whichare calledtrivalent)areboron,aluminum,

and

gallium.

These

atoms form bonds with thesemiconductor atoms, butthebondslackoneelectron fora complete outershellofeight. InFig. 1-9 theimpurity

atom

illustrated

has onlythreevalenceelectrons; therefore,aholeexistsinits

bond

withthe

surrounding atoms. Thus, in acceptordopingholesare introducedintothe

valenceband,so thatconduction

may

occurbythe processofholetransfer.

Since holes can be said to have a positive charge, acceptor-dojjed

semiconductor material is referred to as p-type material.

As

with w-type material,thematerialremainselectricallyneutral,becausethetotal

number

of orbital electrons ineach impurity

atom

isequal tothetotal

number

of

protonsinitsatomicnucleus.Holescanacceptafreeelectron,hencetheterm

acceptordoping.

Even

inintrinsic(undoped) semiconductor materialat

room

(31)

I

Q

J

O

5

®

)

Figure1-9. Acceptor doping.

11

EffectsofHeat andLight

thennal energy causing

some

electrons tobreakthebonds with theiratoms

and

entertheconduction band,socreating pairs of holes

and

electrons.

The

process is termed hole-electronpairgeneration^

and

its converse is a process called recombination.

As

the

name

implies, recombination occurs

when

an

electronfalls intoahole in the vsilence band. Sincethere are

many

more

electrons than holesin n-type material, electronsare said to bethemajority carriers,

and

holesare said tobeminoritycarriers.In/'-typematerialholesare themajority carriers

and

electrons areminoritycarriers.

When

aconductorisheated, theatoms (which areinfixed locations)

tendtovibrate,

and

thevibrationimpedesthe

movement

ofthesurrounding

electrongas.This

means

thatthereisa reductioninthe flow ofthe electrons that constitute theelectriccurrent,

and

we

say that theconductorresistance

hasincreased.

A

conductor has apositive temperaturecoefficient of

resis-tance,i.e.,aresistance

which

increaseswithincresiseintemperature.

When

semiconductor materialisatabsolute zero, there are practically

no

freeelectrons intheconduction

band and no

holesinthevalenceband. Thisisbecauseallelectronsarein

normal

orbit

around

theatoms.Thus,at absolute zero,a semiconductor behavesasaninsulator.

When

thematerialis heated, electrons break

away

from theiratoms

and

move

from thevalence

band

to theconduction band. This producesholesinthevalence

band and

free electrons in the conduction band.

Conduction

can then occur

by

electron

movement

and

byholetrjmsfer. Increasing application ofthermal energy generates

an

increasing

number

ofhole-electronpairs.

As

inthe case

of a conductor, thermal vibration of atoms occurs in a semiconductor.

However,

thereareveryfewelectronstobe imp)edcd

compared

tothedense

electron gas in a conductor.

The

thermal generation of electrons is the

dominatingfactor,

and

thecurrentincrejiseswith temperatureincrease.This

representsa decreaseinsemiconductorresistancevvithtemperatureincrease,

i.e.,anegativetemperaturecoefficient.

An

exceptiontothisruleisheavily

doped

semiconductormaterial,

which

may

behave

more

like a conductor than a semiconductor.

1-10

Effectsof

Heat and

(32)

12

Basic

Just asthermal energy causeselectronstobreaktheiratomic bonds,so

hole-electron pairs

may

be generated

by

energy impartedto the semicon-ductorinthe formof light. Ifthematerialisintrinsic,it

may

have fewfree

electrons

when

notilluminated,

and

thus a very highdarkresutance.

When

illiuninated,itsresistancedecreases

and

may

become comparable

tothat of

a conductor.

1-11

DriftCurrent

and

Diffusion Current

Infreespace,

an

electric field willaccelerate

an

electron inastraight

line from the negative terminal to the positive terminal of the applied

voltage. In a conductor or a semiconductorat

room

temp)erature, a free electron

under

the influence of

an

electric field will

move

towardthejxjsitive

terminal oftheappliedvoltage,but itvvdllcontinuallycollide with atoms along the way.

The

situation is illustrated in Fig. 1-10.

Each

time the electronstrikes

an

atom,itreboundsina

random

direction.

The

presenceof

theelectric fielddoes notstopthecollisions

and

random

motion, butitdoes cause the electron to drift in the direction of the applied electric force.

Current

produced

inthis

way

is

known

asdriftcurrent,

and

itistheusualkind

ofcurrent flowthatoccurs ina conductor.

Figure 1-11 illustratesanother kind ofcurrent.

Suppose

a

concentra-tion ofone typeofchargecarriersoccursatone

end

ofapiece of semicon-ductormaterial.Sincethechargecarriersare eitherallelectronsorallholes,

they have the

same

polarity ofcharge,

and

thus thereisaforce ofrepulsion

between them.

The

resultisthat thereisa tendencyforthechargecarriers to

move

gradually(or diffuse)

from

theregionofhighcarrier densitytoone

oflowdensity. This

movement

continues until all the carriers are evenly

distributed throughout the material.

Any movement

of charge carriers constitutes

an

electric current,

and

this type of

movement

is

known

as

diffusioncurrent.

Both

driftcurrent

and

diffusion currentoccurinsemiconductor

devices.

Electron pathwhen

noelectric fieldispresent

Electronpathwhenthe electric fieldispresent

Conductor Atoms

or

semiconductor

(33)

Charge carrier concentration 13 Glossary of Important Terms

Figure 1-11. Diffusion current.

Nucleus. Centralportion or core of theatom.

Electron.

Very

small negativelychargedparticle.

Electroniccharge. 1.602

X

1 "

"

C.

Proton. Positivelychargedparticlecontainedinthenucleusof

an

atom. Neutron. Particlewith

no

electricalcharge,containedinthenucleusof

an

atom.

Shell.

Path

ofelectron orbiting

around

nucleus.

Atomic

weight. Approximatelythetotalnumb>erofprotons

and

neutrons

inthenucleusof

an

atom.

Atomic number.

The number

ofprotonsor orbiting electronsin

an

atom.

Positive ion.

Atom

thathaslost

an

electron.

Negativeion.

Atom

thathas gained

an

electron.

Germanium

atom.

Atom

ofsemiconductormaterial,hasfour electrons

and

four holes initsoutershell.

Siliconatom.

Atom

ofsemiconductormaterial,hasfour electrons

and

four holes initsoutershell.

Hole.

Absence

ofanelectron

where

one couldexist.

Energy

levelofshell.

Amount

ofenergy required toextracta particular

electronfrom itsatomicshell.

Electron volt (eV).

Energy

required to

move

one electron through a

potential difference of

one

volt.

Energy

band.

Group

ofenergylevelsthat

may

be occupied byelectrons.

Conduction

band.

Energy

band

of electrons that have escaped from

atomicorbits.

Valence

band.

Energy band

of electrons thatarein

normal

atomicorbits.

Forbidden

gap.

Energy

band

at

which

electronsnormally

do

notexist.

Charge

carrier. Electronor hole.

Mobility. Ease(ordifficulty)with

which

achargecarrier

may

be

moved

around.

Conventionalctirrentdirection. Currentflowfrom|X)sitivetonegative.

Electron flowdirection. Electronmotion fromnegativeto p>ositive.

Glossaryof

Important

Terms

(34)

14

Basic

Ionic bond. Electrostatic attraction

when

one

atom

gives

an

electron to another.

Bonding

force in

some

insulators.

Metallic bond. Electrostatic attraction

between

large

numbers

of electrons

and

the

atoms

thathavereleasedthem.

Bonding

force in conductors.

Covalent bond.

Bonding

force thatbinds atoms

which

shareelectrons

and

holes in theiroutershells.

Bonding

force insemiconductors

and

some

insulators.

Electrongas. Large

number

ofelectronsavailable forcurrentcarryingina conductor.

Doping. Additionofimpurity atomsto

change

electricalcharacteristics of

semiconductormaterial.

Donor

atoms. Impurity atoms

which

release additional electrons vkdthin

Acceptor atoms. Impurity

atoms

which

release additional holes within semiconductormaterial.

p-type semiconductor. Semiconductorthathasbeen

doped

with acceptor atoms.

n-tyi>e semiconductor. Semiconductor that has been dopied with

donor

atoms.

Intrinsic.

Name

given to undofsed semiconductor, or tomaterial

doped

equallywithbothtypes of impurities.

Extrinsic.

Name

givento

doped

Majoritycarriers. Typ)eofchargecarriers

which

are in themajorityina given material(electronsinn-type,holes in p-type).

Driftcurrent. Electrons

moving randomly

from one

atom

toanother being

made

todriftinadesired directionunder theinfluence of

an

electric field.

Diffusion current.

Charge

carrier

movement

resulting from

an

initial

concentradonofchargecarriers.

Minoritycarriers. Tyjieofchargecarriers

which

are in theminorityina givenmaterial(holesinn-type,electrons in p-type).

Temperature

coefficient. Ratio of resistance

change

to temperature

change.

Dark

resistance. Resistanceofunilluminated semiconductor.

Crystallattice. Three-dimensional patternin

which atoms

alignthemselves

inasolid.

Hole-electronpair.

A

valence-band hole

and

a

conducdon-band

electron

produced by

energy causingthebreakingofatomic bonds.

Recombination.

Holes

and

electrons recombining, i.e., the

conducdon-band

electronfillsthevalence-bandhole.

Review

1-1. Describe the

atom and draw

a two-dimensional

diagram

toillustrate

Questions

yourdescription.

Compare

the

atom

to a planet wdthorbiting

(35)

1-2.

What

is

meant by

atomicnumber

and

atomic weight? State the atomic 15

number

and

atomic weightforsihcon. Review Questions

1-3.

Name

thethree kindsofbonds that hold

atoms

together inasoHd.

What

kindof

bonding

might be foundin (a)conductors,(b)

insula-tors, (c)semiconductors?

1-4. Explainthebondingprocess insilicon

and germanium. Use

illustra-tions inyour answer.

1-5.

Draw

sketchesto

show

the

bonding

processinconductors

and

insula-tors.

1-6.

What

is

meant

byenergfylevels

and

energy bands?

1-7. Defineconductionband,valenceband,

and

forbiddengap

and

explaintheir origin.

1-8.

Draw

the

band

structure for,

and

explain the difference between, conductors,insulators,

and

semiconductors.

1-9. Defineintrinsicsemiconductors

and

extrinsicsemiconductors.

How

can

extrinsicmaterialbe

made

intrinsic?

1-10.

What

is

meant

by majority carriers

and

minority carriers?

Which

are majoritycarriers

and

why

in(a)donor-dopedmaterial, (b)

acceptor-doped

material?

1-11. Define acceptordoping

and

explain

how

itis effected.

Use

illustra-tionsinyour answer.

1-12.

Repeat

Question1-11 fordonor doping.

1-13.

What

are the

names

givento acceptor-doped material

and

donor-doped

material?Explainwhy.

1-14.

Draw

asketchto

show

the process ofcurrentflowbyhole

movement.

Which

havegreater mobility, electrons or holes?Explain why.

1-15. Explain

what happens

to resistancewithincreasein temf>eraturein

the case of (a)a conductor,(b)a semiconductor,(c)aheavily

doped

semiconductor.

What

do

you think

would

happen

tothe resistance of

an

insulatorwithincreaseintemperature?

Why?

1-16. Explain diffusion current

and

drift current.

Use

illustrations in your answer.

1-17. Explainconventional current direction

and

direction ofelectronmotion.State

why

eachisimportant.

(36)

CHAPTER

2 Junction

Theory

2-1

The

/w-junctionisbasictoallbuta few semiconductordevices.Thus, Introduction itisimportantthat the electronicsstudent gaina thorough understandingof

^-junctiontheory.Thisrequires

an

appreciationofthe forces that act

upon

chargecarrierscrossingthe junction,

and an

understandingoftheeffectsof externally applied bias voltages.

A

knowledge

of thejunction equivalent

circuitsisalsoimportant.

2-2

Figure2-1 representsa/jn-junctionformed by twoblocks of

semicon-Tne

ductormaterial,oneoip-type material

and

theotherofn-type material.

On

pn-junction

^.j^^^-sidethe smallbrokencirclesrepresent holes,

which

arethe majority

carriersinthep-typematerial.

The

dots

on

the n-side representfreeelectrons

within the n-type material.

The

holes

on

the p-side are fixed in position

because the

atoms

in

which

they exist are part of the crystal structure.

Normally

they are uniformly distributed throughout thep-type material.

Similarly,the electrons

on

the n-side areuniformlydistributedthroughout

then-type material.

(37)

17

The pn-)unction

Holes Electrons

Figure2-1. Initialconditionofchargecarriersat pn-junction.

Because holes

and

electronsare close together at thejunction,

some

freeelectronsfromthe n-sideare attracted across thejunction

and

fillholes

on

the/(-side.

They

are said todiffuseacrossthe junction, i.e., flowfrom a

region of high carrier concentration to one of lower concentration (see Section 1-11).

The

free electrons crossing thejunction createnegative ions

on

the /(-side by giving

some

atoms

one

more

electron than their total

number

ofprotons.

They

also leavef)Ositiveionsbehind

them on

the n-side

(atoms with one lesselectronthan the

number

of protons).

The

processis

illustrated inFig.2-2(a).

Beforethechargecarriers diffused acrossthe junction,boththen-type

and

the/)-typ)ematerialwereelectrically neutral.

However,

as negative ions arecreated

on

the/(-sideofthe junction, the region of the/(-sideclosetothe

junction acquires a negative charge. Similarly, the pwsitive ions created

on

the n-side give the n-side a positive charge.

The

accumulated

negative

chargeonthe/(-sidetendstorepelelectrons that are crossingfromthen-side.

(a)Diffusion of charge Holes cross carriersacross (create negativeion) pn-junction.

(b)Junction barrier potentialand

electric field.

Electrons cross (leavepositive ion)

Positivepotential

dueto positive ions

Repels electrons

Electricfield atjunction

(38)

18 and

the

accumulated

positive charge

on

the n-side tends to repel holes P"- crossingfromthe p-side. Thus,it

becomes

difficultfor

more

chargecarriers Theory *° diffuse across the junction.

The

final result is thata barrier potentialis

createdat the junction,negative onthe p-side

and

positive

on

the n-side

[Fig. 2-2(b)].

The

electric field

produced

bythe barrier potentialis large

enough

toprevent

any

further

movement

ofelectrons

and

holes across the

junction.

By

consideringdopingdensities,electroniccharge,

and

temjjerature,it is possible to calculate the

magnitude

of the barrier potential. Typical

barrier potentials at

room

temperature are0.3

V

for

germanium

junctions

and

0.7

V

forsilicon.

The movement

ofchargecarriersacrossthejunctionleavesalayer

on

either side

which

is depleted ofcharge carriers. Thisis thedepletion region

shown

in Fig. 2-3(a).

On

then-side,the depletionregionconsists of

donor

impurity

atoms

which

havelostthefreeelectronassociatedwiththem,

and

havethus

become

positivelycharged.

On

the p-side, theregionis

made

up

of

acceptorimpurityatoms

which

have

become

negatively charged

by

losing

thehole associatedwith

them

(i.e.,theholeisfilled

by an

electron).

On

eachside of the junction,an equal

number

ofimpurityatomsare

involvedin thedepletion region. Ifthe

two

blocks of material haveequal dopingdensities,the depletion layers

on

eachside ofthejunctionareequal

inthickness[Fig.2-3(a)].Ifthe/>-sideis

more

heavily dop)edthanthen-side,

as

shown

inFig.2-3(b), the depletionregionpenetrates

more

deeplyintothe

n-side inordertoinclude

an

equal

number

ofimpurityatoms

on

eachside of the junction. Conversely, if the n-side is the most heavily dojsed, the depletionregionpenetrates deejjer intothej&-typ)ematerial.

It has been

shown

that the electric field produced

by

the barrier

potential atthejunction opf>oses theflowofelectrons fromthe n-side

and

the flow of holesfromthe ^-side. Sinceelectronsare themajority charge

carriers inthe n-typematerial,

and

holesare themajoritychargecarriers in thep-typematerial,itcan be seenthatthe barrierfXJtential opposes theflow of

majoritycarriers.Also,

any

freeelectronsgenerated

on

the p-side

by

thermal energyare attracted across thep)OsitivejXJtentialbarrier to the n-side since electronsare negativelycharged.Similarly,thethermallygeneratedholes

on

the n-sideareattractedtothe p-sidethroughthenegativebarrierpresented

to

them

atthe junction. Electrons

on

the p-side

and

holesonthe n-side are

minority charge carriers.Therefore, the barrier potential assists theflow of

minority carriersacrossthe junction.

To

Summarize:

A

regiondepleted ofcharge carriersspreads acrossboth

sides of a/w-junction,

and

penetrates deejjer into the

more

lightly dof>ed

side.

The

depletionregionencompasses

an

equal

number

ofionizedatomsof oppwsite polarity,

on

opposite sidesofthe junction.

A

barrier potentialexists

due

tothedepletioneffect,positiveonthe n-side

and

negative

on

the p-side ofthe junction.

The

electric fieldfromthe barrier potentialpreventstheflow

(39)

(a)Equaldoping densities Depletion region (~

_O

("i ("p

.+-+ +'

I> (< (1 ii

"+ .+

z

+

~.

O u

'_» < '

+

•J

O

C'

o

\1

Equalnumberof ions

oneach side

Layerof negative ions (depletedof holes)

Layerof positive ions (depletedof electrons)

19

Reverse-Biased Junction Heavilydoped (b)Unequal doping densities '-' i_' ij

_O

',)

+

+ +

\J II

Q

(~i I1

.+~,+

+~ +

' I»

o

n

o O

+

•-+ +

o

o o o

Lightlydoped

Equalnumberof ions

Layerofnegative ions oneach side Layerof positive ions Figure2-3. junctiondepletionregion.

If

an

external bias voltageisappliedfxwitivetothe n-side

and

negative

to thep-sideofa/m-junction, electrons

from

the n-side are attractedtothe positive biasterminal,

and

holesfromthe p-side are attractedtothenegative terminal.Thus, as

shown

inFig. 2-4, holesfromtheimpurityatoms on the p-side of thejunctionare attracted

away

fromthe junction,

and

electrons are attracted

away

fromtheatoms

on

the n-sideofthe junction. Inthis

way

the depletion region is widened,

and

the barrierp>otential isincreased by the

magnitude

of the applied voltage.

With

the barrier potential

and

the

resultant electric field increase, there is no [Xjssibility of majority carrier

currentflow across the junction. Inthiscase, thejunctionissaid tobereverse biased.

Although

there isnopossibility ofa majority carriercurrent flowing

acrossareverse-biasedjunction,minoritycarriersgenerated

on

eachsidecan

2-3

Reverse-Biased Junction

(40)

20

pn-Junction Theory Initialwidthof depletion region

Barrier potential for unbiased junction

Depletion regionwidened byreverse bias voltage

Barrier potential increasedbyreverse bias potential

Figure2-4. Barrierpotentialand depletionregionatreverse-biased junction. still cross the junction. Electrons in the p-side are attracted across the

junctiontothe positive potentialonthe«-side.Holes

on

the n-side

may

be

saidtoflow acrosstothenegativepotentialonthe p-side. Thisis

shown by

thejunction reversecharacteristic, or

graph

of reverse current (I/f) plotted to a baseof reverse voltage

(V,f)(Fig. 2-5).

Only

a very smallreversebiasvoltageis necessary to direct all availableminoritycarriersacross thejunction,

and

when

allminoritycarriersareflowingacross,further increase in biasvoltage

willnotincreasethe current.This currentisreferredtoasareverse saturation current,

and

isdesignatedI^.

Igisnormally a very smallcurrent.Forsilicon,itistypicallylessthan

1 nA, whilefor

germanium

it

may

exceed 10ftA.Thisisbecausethereare

more

minoritychargecarriers availablein

germanium

thaninsilicon,since

chargecarriersare

more

easilydetachedfrom

germanium

atoms.

A

reverse-biased/m-junctioncan be represented

by

a verylarge

resis-tance.

From

Fig. 2-5,itisseen thatwith

5-V

reverse bias

and /y=

10^A,the

reverseresistanceis

5

V

Rfi=—-—

=500 kQ

(41)

Reversebreakdown voltage Reversevoltage 1 Reverse breakdown 10 Reverse current

Figure2-5. pn-junctionreverse characteristics.

For asiliconjunction with

an

/jofabout0.1 /tA

and

areversevoltageof

5 V, R/f is 50 MS2. In practice, the reverse resistance is normally not

specified; instead, theeffect ofreverse saturation current /$ is taken into

accountforeachparticularcircuit.

If the reverse biasvoltage is increased, the velocity of the minority chargecarrierscrossingthejunction isincreased.

These

high-energycharge

carriersstrike the

atoms

within thedepletion region

and

may

cause large

numbers

of charge carriers to be

knocked

out of the atoms {ionization by

collision).

When

thishappens,the

number

ofchargecarriersavalanches,

and

a

large currentflows across the junction. This

phenomenon,

known

asreverse

breakdoum,occursataparticular reverse voltage (thereversebreakdownvoltage)

for agiven/)n-junction (see Fig. 2-5). Unless thecurrent is limited bya

suitableseries resistor,thejunction

may

bedestroyed.Reverse

breakdown

is

employed

ina device

known

asabreakdoum diode,discussedin

Chapter

11.

21

Forward-Biased

Junction

Considertheeffect of

an

external bias voltageappliedwiththe polarity

shown

in Fig.2-6:fxjsitiveonthe/)-side, negativeonthen-side.

The

holeson

the/(-side,being positivelychargedparticles, are repelledfromthe positive biasterminal

and

driventowardthejunction. Similarly, the electrons

on

the n-side are repelled from the negative biasterminal

and

driven towardthe junction.

The

resultisthatthedepletion regionisreducedinwidth,

and

the barrierp>otentialisalsoreduced.Iftheappliedbias voltageisincreasedfrom

zero, the barrier potential gets progressively smaller until it effectively disapf)ears,

and

chargecarrierscaneasily flowacrossthe junction.Electrons

fromthe n-side arethenattracted across to the positive biasterminal

on

the

//-side,

and

holesfromthe/)-sideflow acrosstothenegativeterminalonthe

n-side.Thus, a majoritycarrier currentflows,

and

thejunctionissaidtobe forwardbiased.

2-4

Forward-Biased Junction

(42)

22

pn.

Junction

Theory

Narroweddepletion region

Barrier potential for unbiased junction

Barrierpotential

reducedby forward

biaspotential

Figure2-6. Barrierpotentialatforward-biasedjunction.

Figure 2-7 shows the forward current (Ip) plotted against forward

voltage

(Vp) for typical

germanium and

silcon/)n -junctions.Ineachcase,the

graph

is

known

as theforwardcharacteristicof the (siliconor germanitim)

junction.Itisseen thatverylittleforwardcurrent flows until Vp exceedsthe

junction barrier fwtential (0.3

V

for

germanium,

0.7

V

for silicon).

The

characteristicsfollow

an

exponentiallaw.

As

Vpisincreased to the knee of the characteristic,the barrier potentialisprogressivelyreducedtozero,allowing

(43)

the knee of the characteristic, the barrier potential has been completely overcome,Ipincrejisesalmostlinearlywithincrease in Vp,

and

the

combined

semiconductorblocksaresimplybehavingasaresistor.

Itis obviousthat aforward-biased jimction can be represented by a verylowresistance.

From

p)ointxonFig. 2-7,theforwardresistanceforsilicon iscalculated as 23 Temperature Effects '^ 20

mA

For

germanium,

from pointy

on

Fig.2-7,

0.3

V

R.=

_'

20

mA

=

150

Inpractice,

Rp

isnormallynot used; instead the dynamicresistance(r^)of thejunction is determined. This quantityis also

known

as the incremental resistanceor acresistance.

The

dynamic

resistanceis

measured

asthe reciprocal oftheslofjeoftheforwardcharacteristic

beyond

the knee.

Supf)ose thecurrent

and

voltageconditions are

changed

fromp>ointa

topointb

on

Fig. 2-7.

The

change

inforwardvoltageis

A

F^=^0.1 V,

and

the

change

in forward current is

Alp^^iO

mA,

as illustrated.

The

resistance

change

r^ iscalculated as

^Vp

0.1

V

A/^

40

mA

=

2.5fi

As

discussed in Section 2-3, the reverse current /j is

made

up

of

minority charge carriers crossing the junction.

When

the temperature of

semiconductor material isincreased, the additional thermal energy causes

more

electronsto break

away

from atoms. Thiscreates

more

hole-electron

pairs

and

generates

more

minority chargecarriers.Therefore,/yincreases as

junctiontemperaturerises.

/j can be

shown

to be dejjendent

upon

electronic charge, doping

density,

and

junctionarea, as well as temperature.

With

the exceptionof

temperature,all these factorsarc constant fora givenjunction; thus/y is

altered only by temperature change. It has been found that /j approxi-mately doublesforeach

10°C

increjiseintemperature. Hence, foragfiven

junction, thereisadefinite/j level foreach temperaturelevel(Fig. 2-8).

It hasbeen

shown

that Ig increaseswith increase in temperature. It

can also be

shown

that the forward current Ip is proportional to /j.

Therefore,as illustratedbythe verticalline in Fig.2-9(a),forafixedlevelof

2-5

Temperature

(44)

24

pn-Junction Theory 3 2 /jat25°C

l

/oat45°C

-{-

20

Figure2-8. Temperatureeffectonreverse characteristics. /p at50°C

(45)

Vp,Ipincreases as thejunctiontemperatureincreases.If_/^(atthe increased

^

temfjerature)is

measured

forseverallevelsof Vp

and

theresultsplotted,itis

c i

'""^''°"

seen that the characteristicis

moved

totheleft.

The

horizontalline

on

Fig.

2-9(b)showsthat, ifIpis heldconstant vk-hile thejunction temperatureis

changing, the foward voltage, Vp, decreases with junction temperature

increase(i.e., Vphas a negative temperaturecoefficient). It isfoundthatthe

temperaturecoefficient forthe forwardvoltage ofa /w-junction is approxi-mately

—

1.8

mV/° C

forsilicon

and

—

2.02

mV/°

C

for

germanium.

The

depletion layer of a ^-junction is a region depleted of charge

2-6

carriers.Therefore,as

an

insulatororadielectric

medium

situatedbetween Junction two low-resistance regions,itis acapacitor.

The

valueof thedepletion layer

^3pac

la ce capacitance, designated C,,,

may

becalculatedfromthe usualformula fora

parallel plate capacitor.

A

typicalvalueof

C^

is40picofarads (pF).Since

thewidthofthe depletion layercan be

changed

byalteringthe reverse-bias voltage, the capacitance of a given junction

may

be controlled by the

applied bias. This property is utilized in a variable-capacitance device

known

asavaricaporvaractor(Chapter 19).

Consider a forward-biased junction carrying a current Ip. If the

applied voltage is suddenly reversed, Ip ceasesimmediately, leaving

some

majoritychargecarriersinthe depletion region.

These

chargecarriers

must

flowback outofthe depletion region,

which

is

widened

when

reverse biased.

The

result is that,

when

a forward-biased junction is suddenly reversed,a

reversecurrentflows

which

islargeinitially

and

slowlydecreasestothelevel

of/j.

The

effect

may

belikenedtothedischargingofacapacitor,

and

soitis

representedby a capacitance

known

asthediffusioncapacitance C^. Itcan be

shown

that

Q

is propwrtional to the forward current Ip. This is to be

expected, since the

number

ofchargecarriersin thedepletion region

must

bedirectlyprop>ortionalto Ip.

A

typicalvalueofdiffusioncapacitance C^is

0.02p.?,

which

is very

much

greaterthanthedepletion layercapacitance,

pn

The

effect produced by

Q

is variously

known

as recovery time,carrier storage, or, injunctions with a heavily

doped

/i-region, asholestorage.

The

diffusioncapacitance

becomes

veryimportantindevices

which

arerequired

toswitchrapidlyfrom forwardto reverse bias (seeSection3-11).

A

reverse-biasedjunction can be simply represented as the reverse

2-7

resistance /?„ in parallel with the depletion layer capacitance

C„

lunction

re- o in/

M

Equivalent

[F'g-2-10(a)].

^Circuit

The

equivalentcircuit fora forward-biased junction isrepresentedby

the

dynamic

resistancer, in parallel with the diffusion capacitance

Q. A

battery(torepresent the barrier potential)must be includedin scrieswithr^.

The

complete equivalent circuitfor a forward-biased junctionis

shown

in Fig. 2-10(b).

(46)

26

pn-Junction

Theory

-WW-(a)Equivalentcircuit forreversebiased junction

(b)Equivalentcircuit forforwardbiased junction

Figure 2-10. Equivalentcircuitsfor pn-junction.

Glossary of

Important

Terms

Barrierpotential. Potential ata/w-junction, resulting

from

chargecarriers crossing the junction. Typically, 0.3

V

for

germanium,

0.7

V

for silicon.

Depletionregion.

Narrow

regiondepletedofchargecarriers.

Reverse saturation current. Minority charge carrier current that flows acrossareverse-biased junction.

Avalanche

effect.

Charge

carriersincreasing in

number

by knocking other

chargecarriersoutofatoms.

Reverse

breakdown.

Junction

breakdown under

the influence ofalarge reverse-bias voltage.

Forward

current. Currentthat flows acrossa forward-biased/^-junction.

Depletionlayercapacitance. Junction capacitance

due

todepletion region.

Diffusion capacitance. Junction capacitance

due

toforwardcurrent.

Varicap. Variable capacitancedevice utilizing the depletion layer capaci-tance.

Varactor. Sjimeasvaricap.

Reverseresistance. Resistanceofareverse-biased junction.

Forward

resistance. Resistanceofa forward-biasedjunction.

Reversecharacteristic. Plot of reverse currenttobaseofjunction reverse-bias voltage.