Unit1IntroductiontoSemiconductors

PartI

Semiconductorsarematerialsthatexhibitpropertiesthatarebetweenametalandanon-metal.

Theirelectricalpropertiesarepartwaybetweenaconductorandaninsulator—thustheyarecalled

“semi-conductors”.(Actuallytheyaremuchclosertoinsulatorsthanconductors.)Anumberof

materialshavetheseproperties:

?ThecrystallineformsofmostelementsofGroupIV(valency4)(silicon。！germanium,

tin).

?Someformsofspecificelements—boron(GroupIII),arsenicandantimony(GroupV),

seleniumandtellurium(GroupVI).

?CrystallinealloysofelementsinGroupIII(valency3)withelementsofgroupV(valency

5).Galliumarsenide(acompoundoralloyofgalliumandarsenic)isagoodexamplehere.

?SomecompoundsofelementsofGroupVIwithelementsofGroupII.

Probablythemostversatileelementsarethosewithfourelectronsintheiroutershell(Group

IV:carbon,silicon,germanium,tin,lead).Becauseofthefactthattheycanhavefourbondsto

fourotherelements,covalentcompoundsofendlesscomplexitycanbeformed.(IndeedDNAisa

singlecarbon-basedmolecule.)

Alloftheseelementscanexistinmultiplephysicalforms.Carboncantakemanyformssuch

asgraphite(afbnnabitlikeaclassicalmetal)whichconductselectricityverywellordiamond(a

crystallineform)whichisaninsulator.

Almostanyonewhohaseversolderedanelectricalconnectionknowsthedifferencebetween

agoodsolderedjoint(wherethemetalsolidifiesinapolymorphicform)anda“dryjoint“where

thesoldercrystallizes」?Polymorphicsolderisaverygoodelectricalconductor.Solder(analloy

ofleadandtin)initscrystallineformisasemiconductor!

Infactcarbonisnotclassifiedbychemistsasasemiconductoreventhoughdiamonddoes

semiconductatrelativelyhightemperatures(wellaboveroomtemperature).Thetechnicalreason

isthatcarbonhasverystrongmolecularbondswhichdonotbreakdownveryeasily(theenergy

“gap"betweenthevalencebandandtheconductionbandisgreaterthanthatprovidedbyambient

heatatroomtemperature.)Itwouldbesomewhatdifficultinpracticetousecarbonasabasisfor

semiconductorelectronicssinceitishardtogrowlarge,extremelypurediamondstocutupand

useaschips!

Amajordifferencebetweensemiconductorsandconductorsistheirelectricalbehaviorat

differenttemperatures.Ingeneral,whenyouheatupasemiconductoritsresistancetotheflowof

electricitydecreases(oritsconductanceincreases).Inmetalstheoppositehappens.Asthetempe-

ratureofametalincreasesthenitsresistancealsoincreases(orconductancedecreases).

Todaysiliconisthemostcommonlyusedsemiconductoralthoughgermaniumwasused

almostexclusivelyinearly(1950*s)solidstatedevices.Silicon(likemostelements)cantakea

numberofphysicalforms.Itresemblesametalinitselectricalconductivityproperties.Crystalline

siliconisthebaseforalmostallsemiconductorstoday.

Figure1Siliconcrystallattice

AsiliconcrystallatticeisillustratedinFigure1.Eachsiliconatomisbondedtofourother

siliconatomswithcovalentbonds.Notethatalthoughthepictureistwo-dimensionalthecrystal

structureis(ofcourse)threedimensional.Aperfectsiliconcrystal(atatemperaturenearabsolute

zero)hasnofreeelectronsandthuscannotconductelectricity.Apuresiliconcrystalatanytempe-

ratureaboveabsolutezerowillhavesomebondsbrokenbytherandomactionofheatandsothere

willbesomefreeelectronsinthestructureandyouwillgetsomeconduction.

Inadditiontousingpuresiliconorgermaniuinyoucanalsousecrystalsmadeofalmostany

alloyofelementsinGroupIII(withthreeelectronsintheiroutershell)withelementsinGroupV

(withfiveelectronsintheiroutershell).Thebestknownmaterialusedinthiswayisgallium

arsenidealthoughtherearemanyothers.

ElectricalConductioninanIntrinsicSemiconductor

An“intrinsic“semiconductorisaunifbnnmaterialthatcanactasasemiconductorwithout

theneedtointroduceanomaliesinthestructurebydoping.Purecrystalsofsilicon,germaniumand

galliumarsenideareintrinsicsemiconductorsatroomtemperature.

Dopants

Semiconductorsdon*tgetveryinteresting(oruseful)untilyouintroducedopants.Adopantis

averysmall(indeedminuscule)amountofacontrolledimpurityintroducedintothecrystalline

structure.Thepresenceofthedopantisresponsibleforthepropertiesofthesemiconductor.

Figure2(left)showsasiliconcrystallatticedopedwithboron.Boronhasthreeelectronsinits

outershell.Theregularcrystallinestructureremainsbutnowthereisa"hole"init.Aboronatom

takestheplaceofasiliconatominthestructure.Thestructureistoostrongtobechangedbythe

singleimpurityatomandthuswehaveamissingbondinthestructure.

Figure2(right)showsasiliconcrystaldopedwithphosphorus.Phosphorushasfiveelectrons

initsoutershell.Herethesamethinghappensaswithboronexceptthatwenowhaveanelectron

toomanyratherthananelectrontoofewasfarasthelatticeisconcerned.Thecrystalstructureis

toostrongtobedeformedbytheimpuritybutthereisanelectron“leftover"afterallthebondsare

formed.

Thuswehavetwodifferentkindsofsemiconductingsilicon.Atypewithexcessholesinthe

lattice(calledp-typesilicon)andatypewithexcesselectrons(calledn-typesilicon).

p-typesiliconn-typesilicon

Figure2Dopedsilicon

Newwords

conductor導體

insulator絕緣體

crystalline晶體的

valency化合價

bond結合

covalent共價的

DNA脫氧核糖核酸deoxyribonucleicacid

molecule分子

solder焊料，焊接

polymorphicform多晶型物

atom原子

intrinsic固有的，本質的

doping（半導體）摻雜（質）

dopant摻雜物，摻雜劑

minuscule極小的

hole空穴

Notes

[1]本節中用到的幾種化學元素（物質）名稱：

germanium銘silicon硅tin錫boron硼

selenium硒arsenic幣由tellurium確antimony睇

arsenide神化物gallium短

[2]Almostanyonewhohaseversolderedanelectricalconnectionknowsthedifferencebe-

tweenagoodsolderedjoint(wherethemetalsolidifiesinapolymorphicform)anda"dry

joint”wherethesoldercrystallizes.幾乎所有進行過電子線路焊接的人都知道好焊點

和虛焊點之間的區別，在好的焊點中，金屬凝固成了多晶型物，而在虛焊處，焊料

則是晶體。

Exercise

1.TranslatethefollowingphrasesintoEnglish.

導體/絕緣體電阻/電導原子/分子/離子本征半導體

能級共價鍵金屬/晶體/化合物

2.Answerthequestions.

(I)Carbonisnotclassifiedasasemiconductor.Why?

(2)Whichunlikeelectricalbehavioristherebetweensemiconductorsandconductorsatdiffe-

renttemperatures?

3.TranslatethefollowingsentencesintoChinese.

(1)Infactcarbonisnotclassifiedbychemistsasasemiconductoreventhoughdiamonddoes

semiconductatrelativelyhightemperatures(wellaboveroomtemperature).Thetechnical

reasonisthatcarbonhasverystrongmolecularbondswhichdonotbreakdownvery

easily(theenergy"gap”betweenthevalencebandandtheconductionbandisgreaterthan

thatprovidedbyambientheatatroomtemperature.)

(2)Apuresiliconcrystalatanytemperatureaboveabsolutezerowillhavesomebonds

brokenbytherandomactionofheatandsotherewillbesomefreeelectronsinthestruc-

tureandyouwillgetsomeconduction.

4.TranslatethefollowingsentencesintoEnglish.

(1)半導體的電特性介于導體和絕緣體之間.

(2)一般來說，半導體的電阻隨溫度升高而降低，但導體恰好相反.

PartII

PropertiesofSemiconductors

Thereareanumberofcriticalpointsherewhicharenotallobvious:

N-typesiliconcanconductelectricitybecauseitcontainsfreeandmobileelectrons.Butthe

concentrationofdopantiskeptsoverylowthatthematerialnolongeractsasaconductorbut

4tsemiconducts^^instead.Ifyouincreasetheconcentrationofdopanttoomuchyougetaconductor

asaresultandsincethedevicesweplantomakerelyonsemiconductoreffects,thematerialwould

nolongerwork.

?P-typesiliconcanalsoconductelectricity.Althoughtherearenofreeelectronstheholescan

actasthoughtheyarepositivelychargedparticles.Inrealitytheboundelectronsmigrateinthe

oppositedirectiontoholemovementbutitisaverygoodconceptualizationtoconsidertheholes

asmoving.

?Electricitytravelsconsiderablyfasterinn-typesiliconthanitdoesinp-typesilicon.Free

electronsmovefasterthanholes.

?Electronsinthevalencebandtakecomplexpathsorbitingmanyatomsratherthanstayingin

orbitslinkingpairsofatoms.Thisleadstoaconceptwherethelatticeisboundtogethernotso

muchbyindividualbondsbetweenatomsbutratherbya“cloud“ofelectronswhichacttogetherto

bindthewholestructure.Electronsinvolvedinbonding(withinthevalenceband)arebound

withinthelatticeandcannoteasilyescape.Theymovearoundthestructureonspecific(butinde-

terminate)pathsathighspeed.

Asmentionedbefore,holescanmovearound.Atfirstsight,thisisnotsensible.Aholeisthe

absenceofanelectronataparticularpointinthelattice.Holesareanabsenceofsomethingrather

thanapresenceandquiteobviouslycan'tmoveinthemselves.However,theyappeartomove

becauseelectronsinthelattice,whileconfinedtothebondingstructure(toorbitsaroundatoms),

canjumpfromonepath(aroundonenucleus)tootherpaths.Ifthereisavacantpath(suchas

createdbyahole)itisrelativelyeasyforanelectrontojumpfromanearbyorbitalpathtotakethe

vacantone.Holesdon'tneedtostayanywhereneartheanomalyinthebasicatomicstructure

whichcreatedthembutdotendtostaynearbybecauseofelectrostaticattraction.Aholeisthere

becauseanucleusinthestructurehasavalenceof3ratherthan4andtheatomisheldinsidea

structurewhereit“should“havefour.Iftheholemovesawayfromtheanomalythentheelectron

structureneartheanomalousatomhasoneelectrontoomany.Thusthereisanetnegativecharge

inthisregionandelectronswilltendtoberepelled(orholesattracted).

Freeelectronsareinthe“conduction“band(ahighenergystate)andmovethroughthelattice

viadiffusion.Electronsboundwithinthelatticealsomovebutthesearetravelinginafinite

numberoffixedpossibleorbits.Astheseshiftfromorbittoorbittheholesinthelatticeappearto

moveasthoughtheywerepositivecharges.Holemovementthereforeisreallythemovementof

electronsinthevalenceband(intheoppositedirectiontothenotionalmovementofthehole).

?Itiseasilypossiblefbralatticewithholesinittocoexistwithmobilefreeelectronsinvery

closeproximitywithoutmuchinteractionbetweenthetwo.Thisisanimportantissue.Thewhole

operationofsemiconductordevicesdependsonelectrons“recombining“withholesinmany

situations.Theveryimportantpointisthattheelectronswhichholdthelatticestructuretogether

arenotTherecanbebothfreeelectronsandholespresentincloseproximityatthesame

timebecausetheyrepresentdifferentenergystates.

Whenanelectronintheconductionbandoccupiesaholeinthevalencebandthereisa

releaseofenergy.ThisreleaseofenergyisthebasisfbrtheoperationofLEDsandlasers.

?Whenadopantisusedthequantityisextremelylow.Alevelof1atomin10sisatypical

level.

Togetanideaofwhatthismeans,consideraroom4meterssquareand3metershigh(about

12feetsquareand9feethigh).Ifthisroomwasfilledwithmarblesof1cmdiameterthenitwould

contain144xl06marbles.Ifeachoneoftheserepresentedasingleatom,thenatypicaldopant

levelwouldberepresentedbyasinglemarble.Thusonemarble(atomofdopant)inalargeroom

fullofmarbles(representingatomsofsilicon)illustratesabouttherightproportionofdopant.A

verysmallquantityindeed.

N-typeSilicon

p-typcSiliconn-typeSilicon

一。一。一e十?①?①?①.

①.①?①?①.

①.①?①?①.

①?①?①0

NegativeionHolePositiveionElectron

Figure3Dopedsilicon.P-typchasanexcessofholes(mobilepositive

charges)andn-typchasanexcessofelectrons(mobilenegativecharges).

Considern-typesiliconasillustratedontherightinFigure3.Wehaveanatomofphosphorus

lockedintoasiliconlattice.Whenthelatticestructureofbondsisformedthereisanelectron“left

over".Ofcoursethiselectronisnecessarytobalancethechargebetweenthenucleusofthepho-

sphorusatomandtheelectronsarounditbutthereisnoplacefbritinthebondingstructure.It

becomesfreeandwillwander(veryslowlyandquiterandomly)throughoutthecrystalinfluenced

mainlybythemovementduetoheat.Itwilltendtostayneartheboundphosphorusatom(or

anotherone)becauseofelectrostaticattractiontothenetpositivecharge.However,thisisa

relativelyweakforceandisovercomejustbyenergyimpartedbyheat.

Whentheelectronmovesawayfromitspreviousowner(thephosphorusatom)thenweare

leftwithanatomicstructurewithapositivecharge(itisnolongerbalancedbytheelectronthat

justwanderedaway).Thispositivelychargedatominthelatticeiscalleda"positiveion".When

thelatticeisfirstformedthepositiveionispairedwithafreeelectron.Thepositiveionsconsistof

atomslockedintothestructureinsuchawaythattheycannotmoveinanyway.

Therearealsosomeholespresentinn-typesilicon.Thesearecreatedbytheactionofheaton

theundopedsiliconlattice.Manyofthemwillbefilled(andwillthusdisappear)byrecombination

withtheexcesselectronsinthestructure.Neverthelesstherewillbesomeholes.

P-typeSilicon

P-typesiliconisjustthesameprincipleasn-type.Somedopant(thistimewith3electronsin

itsoutershell)isintroducedintothesilicon.Boronisoftenusedforthispurpose.Theboronatom

becomeslockedintothecrystalstructureandisimmovable.However,boroncannotcontribute4

electronstothebondingprocess—becauseitonlyhasthree.Thismeansthatthereisaholeleftin

thelatticewhereanelectronshouldbebutwhereitisabsent.Atleastthatishowitstartsout.As

emphasizedbefore,theelectronsthatholdthelatticeinplacemovearoundthelatticeveryquickly.

Theytakepathsorbitingmanyoftheatomsinthestructure.Sowhathappensisthattheholeinthe

latticestructureisabletomovethroughthecrystal.Thisresultsintheholebeingabletomove

awayfromtheboronatomwhichwasresponsibleforcreatingit.Thusweareleftwithaplacein

thestructure(aroundtheboronatom)wheretherearetoomanyelectrons(wehavetheright

numberfbrcompletingthelatticebuttoomanytobalancethenumbersofprotonsinthenucleiof

nearbyatoms).Thuswehaveafixed"negativeion”withinthelattice.Sowhenitisfirstformed

westartwithaholeinthelatticearoundtheimpurityboronatombutassoonastheholemigrates

awayweareleftwithanegativeionwheretheholeshouldbeandahole(positivecharge)some-

whereelse.Inp-typesilicontherearesomefreeelectronsavailableinthestructure(onesthatwere

createdbytheactionofambientheatbuthaven*tyetrecombinedwithholes).

MajorityandMinorityCarriers

Inn-typesiliconelectronsarecalledthe“majoritycarriers^^becausetherearefarmoreelec-

tronsavailablethanholes.Inn-typesiliconholesarereferredtoasthe"minoritycarriers”.In

p-typesiliconitistheotherwayaround.Holesbecomethe"majoritycarriers^^andelectronsthe

"minoritycarriers^^.Infurtherdiscussionmosteffectsapplytoeitherp-typeorn-typesiliconbutto

eitherelectronsortoholesdependingonwhichoneisinthemajority.Thuselectronbehaviorin

n-typeandholebehaviorinp-typeisgenericallyreferredtoasmajoritycarrierbehavior.

ElectricalConductioninSemiconductorDevices

Diffusion

Thetermdiffusiondescribesthebehavioroffree(conductionband)electronsandholes

withinasolidwhenthereisnoexternalelectricfieldapplied.Whathappensisthattheelectrons

moveatrandomthroughoutthesolid(sayapieceofmetal)inthesamewayasthemoleculesofa

gasmovewithinaconfinedspacesuchasajar.Eachelectrontakesarandompathbutonaverage

thedensityofelectronsthroughoutthesolidisthesameinallplaces.Thisisbecauseelectrons

carrythesamechargeandhencerepeleachother.Wheretheycanmovefreelythroughasolid

theyfillthewholeavailablespacerelativelyevenly.Gasinajarbehavesinexactlythesameway.

Ifyouhavegasunderpressureinajarandthenopenit,thegaswillpropelitselfoutintothesur-

roundingairveryquickly.Themechanismhereisjustdiffusionatwork.Pressurewilltendtoeven

upoverthewholespaceoccupiedveryquickly.

Drift

Driftisthetermusedtodescribethemovementofelectronsinasemiconductorunderthe

influenceofanelectricfield.Theelectricfieldcausestheelectronstobeacceleratedinthe

directionofthepositivecontact(positiveendofthewire)awayfromthenegativecontact.But

theydon'tgetveryfar.Theyverysoonfindanatominthewayandtheybounceoff(justattheydo

inregulardiffusion).Howevertheexternalforceoftheelectricfieldcausesanetmovementof

electrons(calledcurrent),Agoodwayofthinkingaboutdriftisasdiffusioninaparticular

directionundertheinfluenceofanexternalelectrostaticormagneticfield.

Newwords

concentration濃度

conceptualization概念化

lattice晶格

indeterminate不確定的，模糊的

nucleus[nuclear的復數]核子

anomalous不規則的，反常的

net凈余的

coexist共存

recombine復合

phosphorus磷

crystal晶體

electrostatic靜電的

imparted給予的，授予的

ion離子

proton質子

carrier載流子

diffusion擴散

drift漂移

Unit2SemiconductorDevices

PartI

P-nJunctions

Figure1showsasemiconductorjunctiondiode.Atonesideofthejunctionwehavep-type

siliconandattheothern-type.Themostimportantthingtounderstandaboutsuchajunctionis

thatitisamolecularjunction.Youcannottaketwopiecesofdopedsilicon,polishthemhighlyand

clampthemtogethertoformthiskindofjunction.Thep-typeandn-typeregionsmustbepartof

thesamecontiguoussiliconcrystal.

depletion

p-typezonen-type

。。一。一。一。一。①?①?①?①.

。①?①?①?①.

OoOo0oOoO

一。一。一e。。O①?①?①?①.

一。一。一e。。。①?①?①?①.

Figure1p-njaction

Intheearlydays(1950*s)junctiondiodesandtransistorsweremadebyspotweldingwires

coatedwithdopantontotheoppositefacesofagermaniumcrystal.InsiliconVLSI,dopantsare

diffusedintothecrystalafteritisformedbyusinggaseouscompoundsattemperaturesofaround

1100℃.

Immediatelyafterthejunctionisformedholeswilldiffuseacrossthejunctionfromthep-type

tothen-type.Atthesametimeelectronswilldiffusefromthen-typetothep-type.Sinceholesand

electronsareofoppositechargeandmoveintocloseproximity,manyofthemwillre-combine

withelectronsfillingavailableholes.Thisleavesnegativelychargedionswithinthep-typeand

positivelychargesionswithinthen-type.Thusatthejunctionwehaveanetnegativechargeinthe

p-typesiliconandanetpositivechargeinthen-type.Theregionaroundiscalledthe"depletion

zone“becausechargecarriers(electronsandholes)havebeeneliminated.Thusthezoneis

depletedofchargecarriers(asillustratedinthefigure1).

Asthechargebuildsupnewcarriersfinditdifficulttocrossthejunctionbecausetheyare

repelledbythechargeontheotherside.Thus,moreelectronscan*tcrossthejunctionfromthe

n-typesidebecauseofthenegativechargeonthepside.Likewiseholesfromthep-typesideare

repelledfromthejunctionbythepositivechargeonthen-typeside.

SemiconductorJunctionDiodes

Whenthefieldisappliedinonedirectionthedeviceconductselectricity(calledtheforward

direction),Whenthefieldisappliedintheoppositedirection(thereversedirection)nocurrentcan

flow.

ForwardBias

Whenweconnectanelectricalpotentialacrossthejunctionwiththenegativepoleconnected

tothen-typematerialandthepositivepoleconnectedtothep-typematerialthenthejunction

conducts.

Onthen-typesidefreeelectronsarerepelledfromthecontactandpushedtowardsthe

junction.Onthep-typesideholesarerepelledfromthepositivelychargedcontacttowardsthe

junction.Atthejunctionelectronswillcrossfromthen-typesidetothep-typesideandholeswill

crossfromthep-typesidetothen-typeside.

Assoonastheycross(orperhapsabitbefore)mostholesandelectronswillre-combineand

eliminateeachother.

Whenthishappensthefreeelectronsmustloseaquantumofenergytofilltheavailablehole.

Thisquantumofenergyisradiatedaselectromagneticenergywiththewavelengthdependingon

thesizeoftheenergy“gap”thatthefreeelectroncrosseswhenitfillsthehole.Thisphenomenon

iscalledinjectionluminescence.

IfyouchooseyourmaterialscorrectlythisemitsvisiblelightandyouhavebuiltanLED.

Someelectronsandholes(bychance)don'trecombineandcontinuethroughthematerialuntil

theyreachtheothercontact—butthisisaverysmallnumber.Inthiscontexttheyarecalled

""minoritycarriers^^.

Thekeytooperationhoweveristhatelectronsandholesmustbeabletoleavethecontacts

andenterthesilicon.Thisispossiblebecauseoftheionspresentinthematerial.Onthen-type

side,nearthecontact,thepositivelychargedionprovidesaplaceforanelectronemittedfromthe

contacttoenterthesiliconlattice.Onthep-typeside,thenegativelychargedionshaveanelectron

thatisonlyveryweaklyheldinthelattice.Thiselectroniseasilyattractedoutofthelatticeonto

thepositivecontactandthusanewholeinthelatticeisborn.

Thuselectronsenterthen-typematerialatthecontactandflowtothejunction.Holesare

createdatthecontactinthep-typematerial(bylossofelectronstothepositivecontact)andflow

tothejunction.Holesandelectronscombineandareannihilatedatthejunction.

Thuselectricityflowsthroughthedevice.

ReverseBias

Whenavoltageisappliedinthe“reverse“directionnocurrentflowsatall.Anegativecharge

isappliedtothecontactonthep-typesideandapositivechargeisappliedtothecontactonthe

n-typeside.Inthiscasethecontactsbothattractthemobilecharges.Onthen-typesidethemobile

electronsareattractedtothepositivecontactandonthep-typesideholesareattractedtothe

negativechargeonthecontact.Thusthedepletionzoneenlargesandthereisnoconduction.

Thereishoweverasmallcurrentcausedbytherandomionizationofcovalentbondswithin

thedepletionzone.Heatcausestherandombreakingofabondcreatingbothaholeandafree

electron.Thefreeelectronisattractedbytheelectricfieldtowardsthepositivecontactandthe

holeisattractedtowardsthenegativecontact.Thefreecarrierscancrossthejunctionifnecessary.

Thisprocessiscontinuousatroomtemperaturethusthereisasmallcurrent.Thiscurrentis

independentoftheappliedvoltagebutvarieswithtemperature.

Newwords

contiguous鄰近的，接近的

transistor晶體管

VLSI超大規模集成電路

depletionzone耗盡層

quantum量子，量子論

luminescence發光

annihilate消滅，殲滅

PartII

TheBipolarJunctionTransistor(BJT)⑴

Inthebeginningtherewasthe“transistor”.Althoughthereweremanydifferenttypestheyall

usedthesamebasicprinciple.Later,anewtypeoftransistoroperatingbyadifferentprinciplewas

invented.InordertodistinguishbetweenthetwotypesthetermuBi-PolarJunctionTransistor”

(BJT)wascoinedtorefertotheearliertype.Thenewfamilyoftransistordevicesisgenerically

calledthe“FieldEffectTransistor^^(FET).

TheBJTisbasicallyanamplifierofelectricalcurrent.Thatis,variationsofcurrentinone

circuitarerepeatedaslargercurrentvariationsinadifferentcircuit.Ofcoursedependingonthe

wayitisconnectedintoanelectricalcircuit,theBJTcanbeusedasavoltageamplifier.However,

thebasicprocessinvolvedisoneofcurrentamplification.

EmitterEmitter

Figure2BasicBITconfigurations

ABJTconsistsoftwosemiconductorp-njunctionsconnectedback-to-back.Figure2shows

schematicsofthetwotypesofBJT.Thesearereferredtoasn-p-nandp-n-pconfigurationsde-

pendingonthetypeofdopedsiliconused.Aswillbeseenp-n-pandn-p-ntransistorshavesimilar

characteristicsbutthepolarityofthecurrentsmustbereversed(becausethematerialtypesused

arereversed).

Itiseasytobemisledherebythewayinwhichtheschematicsareusuallydrawn.Thewhole

operationoftheBJTreliesonthefactthatthe"base"regionisverythin.Therearethreedoped

regionswhichhavedifferentfunctions.Thesearecalledthe“emitter","base"andthe“conector”.0

Fromthediagramitseemsthattheemitterandthecollectorareinterchangeable.Toapointthisis

true.Manycommoditydiscretecomponenttransistorscanbeusedinjustthiswaybutthereare

differencesinpractice

?Theemitterregionisusuallyquitesmallinareacomparedtothecollector.Alsoitisusually

heavilydopedtoprovideagoodsupplyofchargecarriers.

?Thebase,asmentionedbeforeisverythin.

?Thecollectorisoftenquitelargeinordertodissipatetheheat.

WhenimplementedinVLSItechnologyaBJTlooksalotdifferentfromitsrealizationasa

free-standingdevice.Thisisbecauselargenumbersofthemhavetobebuiltonasinglesubstrate

bylithogr