拉扎维授课课件 Ch1-8

FundamentalsofMicroelectronicsCH1WhyMicroelectronics?CH2BasicPhysicsofSemiconductorsCH3DiodeCircuitsCH4PhysicsofBipolarTransistorsCH5BipolarAmplifiersCH6PhysicsofMOSTransistorsCH7CMOSAmplifiersCH8OperationalAmplifierAsABlackBox1

1.1ElectronicsversusMicroelectronics1.2ExampleofElectronicSystem:CellularTelephone1.3AnalogversusDigital

Chapter1WhyMicroelectronics?2CellularTechnologyAnimportantexampleofmicroelectronics.Microelectronicsexistinblackboxesthatprocessthereceivedandtransmittedvoicesignals.3CH1WhyMicroelectronics?FrequencyUp-conversionVoiceis“up-converted”bymultiplyingtwosinusoids.Whenmultiplyingtwosinusoidsintimedomain,theirspectraareconvolvedinfrequencydomain.4CH1WhyMicroelectronics?TransmitterTwofrequenciesaremultipliedandradiatedbyanantennain(a).Apoweramplifierisaddedin(b)toboostthesignal.5CH1WhyMicroelectronics?ReceiverHighfrequencyistranslatedtoDCbymultiplyingbyfC.Alow-noiseamplifierisneededforsignalboostingwithoutexcessivenoise.6CH1WhyMicroelectronics?DigitalorAnalog?X1(t)isoperatingat100Mb/sandX2(t)isoperatingat1Gb/s.Adigitalsignaloperatingatveryhighfrequencyisvery“analog”.7CH1WhyMicroelectronics?Chapter2BasicPhysicsofSemiconductors2.1Semiconductormaterialsandtheirproperties2.2PN-junctiondiodes2.3ReverseBreakdown8SemiconductorPhysicsSemiconductordevicesserveasheartofmicroelectronics.PNjunctionisthemostfundamentalsemiconductordevice.9CH2BasicPhysicsofSemiconductorsChargeCarriersinSemiconductorTounderstandPNjunction’sIVcharacteristics,itisimportanttounderstandchargecarriers’behaviorinsolids,howtomodifycarrierdensities,anddifferentmechanismsofchargeflow.10CH2BasicPhysicsofSemiconductorsPeriodicTableThisabridgedtablecontainselementswiththreetofivevalenceelectrons,withSibeingthemostimportant.11CH2BasicPhysicsofSemiconductorsSiliconSihasfourvalenceelectrons.Therefore,itcanformcovalentbondswithfourofitsneighbors.Whentemperaturegoesup,electronsinthecovalentbondcanbecomefree.

12CH2BasicPhysicsofSemiconductorsElectron-HolePairInteractionWithfreeelectronsbreakingoffcovalentbonds,holesaregenerated.Holescanbefilledbyabsorbingotherfreeelectrons,soeffectivelythereisaflowofchargecarriers.13CH2BasicPhysicsofSemiconductorsFreeElectronDensityataGivenTemperatureEg,orbandgapenergydetermineshowmucheffortisneededtobreakoffanelectronfromitscovalentbond.Thereexistsanexponentialrelationshipbetweenthefree-electrondensityandbandgapenergy.14CH2BasicPhysicsofSemiconductorsDoping(Ntype)PureSicanbedopedwithotherelementstochangeitselectricalproperties.Forexample,ifSiisdopedwithP(phosphorous),thenithasmoreelectrons,orbecomestypeN(electron).

15CH2BasicPhysicsofSemiconductorsDoping(Ptype)IfSiisdopedwithB(boron),thenithasmoreholes,orbecomestypeP.

16CH2BasicPhysicsofSemiconductorsSummaryofChargeCarriers17CH2BasicPhysicsofSemiconductorsElectronandHoleDensitiesTheproductofelectronandholedensitiesisALWAYSequaltothesquareofintrinsicelectrondensityregardlessofdopinglevels.

MajorityCarriers:MinorityCarriers:MajorityCarriers:MinorityCarriers:18CH2BasicPhysicsofSemiconductorsFirstChargeTransportationMechanism:DriftTheprocessinwhichchargeparticlesmovebecauseofanelectricfieldiscalleddrift.Chargeparticleswillmoveatavelocitythatisproportionaltotheelectricfield.19CH2BasicPhysicsofSemiconductorsCurrentFlow:GeneralCaseElectriccurrentiscalculatedastheamountofchargeinvmetersthatpassesthruacross-sectionifthechargetravelwithavelocityofvm/s.20CH2BasicPhysicsofSemiconductorsCurrentFlow:DriftSincevelocityisequaltoE,driftcharacteristicisobtainedbysubstitutingVwithEinthegeneralcurrentequation.Thetotalcurrentdensityconsistsofbothelectronsandholes.21CH2BasicPhysicsofSemiconductorsVelocitySaturationAtopictreatedinmoreadvancedcoursesisvelocitysaturation.Inreality,velocitydoesnotincreaselinearlywithelectricfield.Itwilleventuallysaturatetoacriticalvalue.22CH2BasicPhysicsofSemiconductorsSecondChargeTransportationMechanism:DiffusionChargeparticlesmovefromaregionofhighconcentrationtoaregionoflowconcentration.Itisanalogoustoaneverydayexampleofaninkdropletinwater.

23CH2BasicPhysicsofSemiconductorsCurrentFlow:DiffusionDiffusioncurrentisproportionaltothegradientofcharge(dn/dx)alongthedirectionofcurrentflow.Itstotalcurrentdensityconsistsofbothelectronsandholes.24CH2BasicPhysicsofSemiconductorsExample:Linearvs.NonlinearChargeDensityProfileLinearchargedensityprofilemeansconstantdiffusioncurrent,whereasnonlinearchargedensityprofilemeansvaryingdiffusioncurrent.

25CH2BasicPhysicsofSemiconductorsEinstein'sRelationWhiletheunderlyingphysicsbehinddriftanddiffusioncurrentsaretotallydifferent,Einstein’srelationprovidesamysteriouslinkbetweenthetwo.26CH2BasicPhysicsofSemiconductorsPNJunction(Diode)WhenN-typeandP-typedopantsareintroducedside-by-sideinasemiconductor,aPNjunctionoradiodeisformed.27CH2BasicPhysicsofSemiconductorsDiode’sThreeOperationRegionsInordertounderstandtheoperationofadiode,itisnecessarytostudyitsthreeoperationregions:equilibrium,reversebias,andforwardbias.28CH2BasicPhysicsofSemiconductorsCurrentFlowAcrossJunction:DiffusionBecauseeachsideofthejunctioncontainsanexcessofholesorelectronscomparedtotheotherside,thereexistsalargeconcentrationgradient.Therefore,adiffusioncurrentflowsacrossthejunctionfromeachside.

29CH2BasicPhysicsofSemiconductorsDepletionRegionAsfreeelectronsandholesdiffuseacrossthejunction,aregionoffixedionsisleftbehind.Thisregionisknownasthe“depletionregion.”

30CH2BasicPhysicsofSemiconductorsCurrentFlowAcrossJunction:DriftThefixedionsindepletionregioncreateanelectricfieldthatresultsinadriftcurrent.31CH2BasicPhysicsofSemiconductorsCurrentFlowAcrossJunction:EquilibriumAtequilibrium,thedriftcurrentflowinginonedirectioncancelsoutthediffusioncurrentflowingintheoppositedirection,creatinganetcurrentofzero.ThefigureshowsthechargeprofileofthePNjunction.32CH2BasicPhysicsofSemiconductorsBuilt-inPotentialBecauseoftheelectricfieldacrossthejunction,thereexistsabuilt-inpotential.Itsderivationisshownabove.33CH2BasicPhysicsofSemiconductorsDiodeinReverseBiasWhentheN-typeregionofadiodeisconnectedtoahigherpotentialthantheP-typeregion,thediodeisunderreversebias,whichresultsinwiderdepletionregionandlargerbuilt-inelectricfieldacrossthejunction.34CH2BasicPhysicsofSemiconductorsReverseBiasedDiode’sApplication:Voltage-DependentCapacitorThePNjunctioncanbeviewedasacapacitor.ByvaryingVR,thedepletionwidthchanges,changingitscapacitancevalue;therefore,thePNjunctionisactuallyavoltage-dependentcapacitor.35CH2BasicPhysicsofSemiconductorsVoltage-DependentCapacitanceTheequationsthatdescribethevoltage-dependentcapacitanceareshownabove.

36CH2BasicPhysicsofSemiconductorsVoltage-ControlledOscillatorAveryimportantapplicationofareverse-biasedPNjunctionisVCO,inwhichanLCtankisusedinanoscillator.BychangingVR,wecanchangeC,whichalsochangestheoscillationfrequency.

37CH2BasicPhysicsofSemiconductorsDiodeinForwardBiasWhentheN-typeregionofadiodeisatalowerpotentialthantheP-typeregion,thediodeisinforwardbias.Thedepletionwidthisshortenedandthebuilt-inelectricfielddecreased.38CH2BasicPhysicsofSemiconductorsMinorityCarrierProfileinForwardBiasUnderforwardbias,minoritycarriersineachregionincreaseduetotheloweringofbuilt-infield/potential.Therefore,diffusioncurrentsincreasetosupplytheseminoritycarriers.39CH2BasicPhysicsofSemiconductorsDiffusionCurrentinForwardBiasDiffusioncurrentwillincreaseinordertosupplytheincreaseinminoritycarriers.Themathematicsareshownabove.40CH2BasicPhysicsofSemiconductorsMinorityChargeGradientMinoritychargeprofileshouldnotbeconstantalongthex-axis;otherwise,thereisnoconcentrationgradientandnodiffusioncurrent.RecombinationoftheminoritycarrierswiththemajoritycarriersaccountsforthedroppingofminoritycarriersastheygodeepintothePorNregion.41CH2BasicPhysicsofSemiconductorsForwardBiasCondition:SummaryInforwardbias,therearelargediffusioncurrentsofminoritycarriersthroughthejunction.However,aswegodeepintothePandNregions,recombinationcurrentsfromthemajoritycarriersdominate.Thesetwocurrentsadduptoaconstantvalue.42CH2BasicPhysicsofSemiconductorsIVCharacteristicofPNJunctionThecurrentandvoltagerelationshipofaPNjunctionisexponentialinforwardbiasregion,andrelativelyconstantinreversebiasregion.43CH2BasicPhysicsofSemiconductorsParallelPNJunctionsSincejunctioncurrentsareproportionaltothejunction’scross-sectionarea.TwoPNjunctionsputinparallelareeffectivelyonePNjunctionwithtwicethecross-sectionarea,andhencetwicethecurrent.44CH2BasicPhysicsofSemiconductorsConstant-VoltageDiodeModelDiodeoperatesasanopencircuitifVD<VD,onandaconstantvoltagesourceofVD,onifVDtendstoexceedVD,on.45CH2BasicPhysicsofSemiconductorsExample:DiodeCalculationsThisexampleshowsthesimplicityprovidedbyaconstant-voltagemodeloveranexponentialmodel.Foranexponentialmodel,iterativemethodisneededtosolveforcurrent,whereasconstant-voltagemodelrequiresonlylinearequations.forfor46CH2BasicPhysicsofSemiconductorsReverseBreakdownWhenalargereversebiasvoltageisapplied,breakdownoccursandanenormouscurrentflowsthroughthediode.47CH2BasicPhysicsofSemiconductorsZenervs.AvalancheBreakdownZenerbreakdownisaresultofthelargeelectricfieldinsidethedepletionregionthatbreakselectronsorholesofftheircovalentbonds.Avalanchebreakdownisaresultofelectronsorholescollidingwiththefixedionsinsidethedepletionregion.48CH2BasicPhysicsofSemiconductorsChapter3DiodeCircuits3.1IdealDiode3.2PNJunctionasaDiode3.3ApplicationsofDiodes49DiodeCircuitsAfterwehavestudiedindetailthephysicsofadiode,itistimetostudyitsbehaviorasacircuitelementanditsmanyapplications.50CH3DiodeCircuitsDiode’sApplication:CellPhoneChargerAnimportantapplicationofdiodeischargers.Diodeactsastheblackbox(aftertransformer)thatpassesonlythepositivehalfofthestepped-downsinusoid.51CH3DiodeCircuitsDiode’sActioninTheBlackBox(IdealDiode)Thediodebehavesasashortcircuitduringthepositivehalfcycle(voltageacrossittendstoexceedzero),andanopencircuitduringthenegativehalfcycle(voltageacrossitislessthanzero).

52CH3DiodeCircuitsIdealDiodeInanidealdiode,ifthevoltageacrossittendstoexceedzero,currentflows.Itisanalogoustoawaterpipethatallowswatertoflowinonlyonedirection.53CH3DiodeCircuitsDiodesinSeriesDiodescannotbeconnectedinseriesrandomly.Forthecircuitsabove,onlya)canconductcurrentfromAtoC.54CH3DiodeCircuitsIVCharacteristicsofanIdealDiodeIfthevoltageacrossanodeandcathodeisgreaterthanzero,theresistanceofanidealdiodeiszeroandcurrentbecomesinfinite.However,ifthevoltageislessthanzero,theresistancebecomesinfiniteandcurrentiszero.55CH3DiodeCircuitsAnti-ParallelIdealDiodesIftwodiodesareconnectedinanti-parallel,itactsasashortforallvoltages.56CH3DiodeCircuitsDiode-ResistorCombinationTheIVcharacteristicofthisdiode-resistorcombinationiszerofornegativevoltagesandOhm’slawforpositivevoltages.57CH3DiodeCircuitsDiodeImplementationofORGateThecircuitaboveshowsanexampleofdiode-implementedORgate.VoutcanonlybeeitherVAorVB,notboth.58CH3DiodeCircuitsInput/OutputCharacteristicsWhenVinislessthanzero,thediodeopens,soVout=Vin.WhenVinisgreaterthanzero,thediodeshorts,soVout=0.

59CH3DiodeCircuitsDiode’sApplication:RectifierArectifierisadevicethatpassespositive-halfcycleofasinusoidandblocksthenegativehalf-cycleorviceversa.WhenVinisgreaterthan0,diodeshorts,soVout=Vin;however,whenVinislessthan0,diodeopens,nocurrentflowsthruR1,Vout=IR1R1=0.60CH3DiodeCircuitsSignalStrengthIndicatorTheaveragedvalueofarectifieroutputcanbeusedasasignalstrengthindicatorfortheinput,sinceVout,avgisproportionaltoVp,theinputsignal’samplitude.forfor61CH3DiodeCircuitsDiode’sapplication:LimiterThepurposeofalimiteristoforcetheoutputtoremainbelowcertainvalue.Ina),theadditionofa1VbatteryforcesthediodetoturnonafterV1hasbecomegreaterthan1V.62CH3DiodeCircuitsLimiter:WhenBatteryVariesAninterestingcaseoccurswhenVB(battery)varies.RectificationfailsifVBisgreaterthantheinputamplitude.63CH3DiodeCircuitsDifferentModelsforDiodeSofarwehavestudiedtheidealmodelofdiode.However,therearestilltheexponentialandconstantvoltagemodels.64CH3DiodeCircuitsInput/OutputCharacteristicswithIdealandConstant-VoltageModelsThecircuitaboveshowsthedifferencebetweentheidealandconstant-voltagemodel;thetwomodelsyieldtwodifferentbreakpointsofslope.65CH3DiodeCircuitsInput/OutputCharacteristicswithaConstant-VoltageModelWhenusingaconstant-voltagemodel,thevoltagedropacrossthediodeisnolongerzerobutVd,onwhenitconducts.66CH3DiodeCircuitsAnotherConstant-VoltageModelExampleInthisexample,sinceVinisconnectedtothecathode,thediodeconductswhenVinisverynegative.ThebreakpointwheretheslopechangesiswhenthecurrentacrossR1isequaltothecurrentacrossR2.67CH3DiodeCircuitsExponentialModelInthisexample,sincethetwodiodeshavedifferentcross-sectionareas,onlyexponentialmodelcanbeused.ThetwocurrentsaresolvedbysummingthemwithIin,andequatingtheirvoltages.68CH3DiodeCircuitsAnotherConstant-VoltageModelExampleThisexampleshowstheimportanceofgoodinitialguessandcarefulconfirmation.

69CH3DiodeCircuitsCellPhoneAdapterVout=3VD,onisusedtochargecellphones.However,ifIxchanges,iterativemethodisoftenneededtoobtainasolution,thusmotivatingasimplertechnique.Ix70CH3DiodeCircuitsSmall-SignalAnalysisSmall-signalanalysisisperformedaroundabiaspointbyperturbingthevoltagebyasmallamountandobservingtheresultinglinearcurrentperturbation.71CH3DiodeCircuitsSmall-SignalAnalysisinDetailIftwopointsontheIVcurveofadiodearecloseenough,thetrajectoryconnectingthefirsttothesecondpointislikealine,withtheslopebeingtheproportionalityfactorbetweenchangeinvoltageandchangeincurrent.

72CH3DiodeCircuitsSmall-SignalIncrementalResistanceSincethere’salinearrelationshipbetweenthesmallsignalcurrentandvoltageofadiode,thediodecanbeviewedasalinearresistorwhenonlysmallchangesareofinterest.73CH3DiodeCircuitsSmallSinusoidalAnalysisIfasinusoidalvoltagewithsmallamplitudeisapplied,theresultingcurrentisalsoasmallsinusoidaroundaDCvalue.74CH3DiodeCircuitsCauseandEffectIn(a),voltageisthecauseandcurrentistheeffect.In(b),theotherwayaround.75CH3DiodeCircuitsAdapterExampleRevisitedWithourunderstandingofsmall-signalanalysis,wecanrevisitourcellphonechargerexampleandeasilysolveitwithjustalgebrainsteadofiterations.76CH3DiodeCircuitsSimpleisBeautifulIn

thisexamplewestudytheeffectofcellphonepullingsomecurrentfromthediodes.Usingsmallsignalanalysis,thisiseasilydone.However,imaginethenightmare,ifweweretosolveitusingnon-linearequations.

77CH3DiodeCircuitsApplicationsofDiode78CH3DiodeCircuitsHalf-WaveRectifierAverycommonapplicationofdiodesishalf-waverectification,whereeitherthepositiveornegativehalfoftheinputisblocked.But,howdowegenerateaconstantoutput?79CH3DiodeCircuitsDiode-CapacitorCircuit:ConstantVoltageModelIftheresistorinhalf-waverectifierisreplacedbyacapacitor,afixedvoltageoutputisobtainedsincethecapacitor(assumedideal)hasnopathtodischarge.

80CH3DiodeCircuitsDiode-CapacitorCircuit:IdealModelNotethat(b)isjustlikeVin,onlyshifteddown.81CH3DiodeCircuitsDiode-CapacitorWithLoadResistorApathisavailableforcapacitortodischarge.Therefore,Voutwillnotbeconstantandarippleexists.

82CH3DiodeCircuitsBehaviorforDifferentCapacitorValuesForlargeC1,Vouthassmallripple.83CH3DiodeCircuitsPeaktoPeakamplitudeofRippleTherippleamplitudeisthedecayingpartoftheexponential.Ripplevoltagebecomesaproblemifitgoesabove5to10%oftheoutputvoltage.84CH3DiodeCircuitsMaximumDiodeCurrentThediodehasitsmaximumcurrentatt1,sincethat’swhentheslopeofVoutisthegreatest.Thiscurrenthastobecarefullycontrolledsoitdoesnotdamagethedevice.85CH3DiodeCircuitsFull-WaveRectifierAfull-waverectifierpassesboththenegativeandpositivehalfcyclesoftheinput,whileinvertingthenegativehalfoftheinput.Asprovedlater,afull-waverectifierreducestheripplebyafactoroftwo.

86CH3DiodeCircuitsTheEvolutionofFull-WaveRectifierFigures(e)and(f)showthetopologythatinvertsthenegativehalfcycleoftheinput.87CH3DiodeCircuitsFull-WaveRectifier:BridgeRectifierThefigureaboveshowsafull-waverectifier,whereD1andD2pass/invertthenegativehalfcycleofinputandD3andD4passthepositivehalfcycle.88CH3DiodeCircuitsInput/OutputCharacteristicsofaFull-WaveRectifier(Constant-VoltageModel)Thedead-zonearoundVinarisesbecauseVinmustexceed2VD,ONtoturnonthebridge.

89CH3DiodeCircuitsCompleteFull-WaveRectifierSinceC1onlygets½ofperiodtodischarge,ripplevoltageisdecreasedbyafactorof2.Also(b)showsthateachdiodeissubjectedtoapproximatelyoneVpreversebiasdrop(versus2Vpinhalf-waverectifier).90CH3DiodeCircuitsCurrentCarriedbyEachDiodeintheFull-WaveRectifier91CH3DiodeCircuitsSummaryofHalfandFull-WaveRectifiersFull-waverectifierismoresuitedtoadapterandchargerapplications.92CH3DiodeCircuitsVoltageRegulatorTheripplecreatedbytherectifiercanbeunacceptabletosensitiveload;therefore,aregulatorisrequiredtoobtainaverystableoutput.Threediodesoperateasaprimitiveregulator.93CH3DiodeCircuitsVoltageRegulationWithZenerDiodeVoltageregulationcanbeaccomplishedwithZenerdiode.Sincerdissmall,largechangeintheinputwillnotbereflectedattheoutput.94CH3DiodeCircuitsLineRegulationVS.LoadRegulationLineregulationisthesuppressionofchangeinVoutduetochangeinVin(b).LoadregulationisthesuppressionofchangeinVoutduetochangeinloadcurrent(c).

95CH3DiodeCircuitsEvolutionofAC-DCConverter96CH3DiodeCircuitsLimitingCircuitsThemotivationofhavinglimitingcircuitsistokeepthesignalbelowathresholdsoitwillnotsaturatetheentirecircuitry.Whenareceiverisclosetoabasestation,signalsarelargeandlimitingcircuitsmayberequired.

97CH3DiodeCircuitsInput/OutputCharacteristicsNotetheclippingoftheoutputvoltage.98CH3DiodeCircuitsLimitingCircuitUsingaDiode:

PositiveCycleClippingAswasstudiedinthepast,thecombinationofresistor-diodecreateslimitingeffect.99CH3DiodeCircuitsLimitingCircuitUsingaDiode:

NegativeCycleClipping100CH3DiodeCircuitsLimitingCircuitUsingaDiode:

PositiveandNegativeCycleClipping101CH3DiodeCircuitsGeneralVoltageLimitingCircuitTwobatteriesinserieswiththeantiparallediodescontrolthelimitingvoltages.

102CH3DiodeCircuitsNon-idealitiesinLimitingCircuitsTheclippingregionisnotexactlyflatsinceasVinincreases,thecurrentsthroughdiodeschange,andsodoesthevoltagedrop.103CH3DiodeCircuitsCapacitiveDivider104CH3DiodeCircuitsWaveformShifter:Peakat-2VpAsVinincreases,D1turnsonandVoutiszero.AsVindecreases,D1turnsoff,andVoutdropswithVinfromzero.ThelowestVoutcangois-2Vp,doublingthevoltage.105CH3DiodeCircuitsWaveformShifter:Peakat2VpSimilarly,whentheterminalsofthediodeareswitched,avoltagedoublerwithpeakvalueat2Vpcanbeconceived.106CH3DiodeCircuitsVoltageDoublerTheoutputincreasesbyVp,Vp/2,Vp/4,etcineachinputcycle,eventuallysettlingto2Vp.107CH3DiodeCircuitsCurrentthruD1inVoltageDoubler108CH3DiodeCircuitsAnotherApplication:VoltageShifter109CH3DiodeCircuitsVoltageShifter(2VD,ON)110CH3DiodeCircuitsDiodeasElectronicSwitchDiodeasaswitchfindsapplicationinlogiccircuitsanddataconverters.111CH3DiodeCircuitsJunctionFeedthroughForthecircuitshowninparte)ofthepreviousslide,asmallfeedthroughfrominputtooutputviathejunctioncapacitorsexistsevenifthediodesarereversebiasedTherefore,C1hastobelargeenoughtominimizethisfeedthrough.112CH3DiodeCircuitsChapter4

PhysicsofBipolarTransistors4.1GeneralConsiderations4.2StructureofBipolarTransistor4.3OperationofBipolarTransistorinActiveMode4.4BipolarTransistorModels4.5OperationofBipolarTransistorinSaturationMode4.6ThePNPTransistor113BipolarTransistorInthechapter,wewillstudythephysicsofbipolartransistorandderivelargeandsmallsignalmodels.114CH4PhysicsofBipolarTransistorsVoltage-DependentCurrentSourceAvoltage-dependentcurrentsourcecanactasanamplifier.IfKRLisgreaterthan1,thenthesignalisamplified.115CH4PhysicsofBipolarTransistorsVoltage-DependentCurrentSourcewithInputResistanceRegardlessoftheinputresistance,themagnitudeofamplificationremainsunchanged.116CH4PhysicsofBipolarTransistorsExponentialVoltage-DependentCurrentSourceAthree-terminalexponentialvoltage-dependentcurrentsourceisshownabove.Ideally,bipolartransistorcanbemodeledassuch.117CH4PhysicsofBipolarTransistorsStructureandSymbolofBipolarTransistorBipolartransistorcanbethoughtofasasandwichofthreedopedSiregions.Theoutertworegionsaredopedwiththesamepolarity,whilethemiddleregionisdopedwithoppositepolarity.118CH4PhysicsofBipolarTransistorsInjectionofCarriersReversebiasedPNjunctioncreatesalargeelectricfieldthatsweepsanyinjectedminoritycarrierstotheirmajorityregion.Thisabilityprovesessentialintheproperoperationofabipolartransistor.119CH4PhysicsofBipolarTransistorsForwardActiveRegionForwardactiveregion:VBE>0,VBC<0.Figureb)presentsawrongwayofmodelingfigurea).120CH4PhysicsofBipolarTransistorsAccurateBipolarRepresentationCollectoralsocarriescurrentduetocarrierinjectionfrombase.121CH4PhysicsofBipolarTransistorsCarrierTransportinBase122CH4PhysicsofBipolarTransistorsCollectorCurrentApplyingthelawofdiffusion,wecandeterminethechargeflowacrossthebaseregionintothecollector.Theequationaboveshowsthatthetransistorisindeedavoltage-controlledelement,thusagoodcandidateasanamplifier.

123CH4PhysicsofBipolarTransistorsParallelCombinationofTransistorsWhentwotransistorsareputinparallelandexperiencethesamepotentialacrossallthreeterminals,theycanbethoughtofasasingletransistorwithtwicetheemitterarea.124CH4PhysicsofBipolarTransistorsSimpleTransistorConfigurationAlthoughatransistorisavoltagetocurrentconverter,outputvoltagecanbeobtainedbyinsertingaloadresistorattheoutputandallowingthecontrolledcurrenttopassthruit.125CH4PhysicsofBipolarTransistorsConstantCurrentSourceIdeally,thecollectorcurrentdoesnotdependonthecollectortoemittervoltage.Thispropertyallowsthetransistortobehaveasaconstantcurrentsourcewhenitsbase-emittervoltageisfixed.126CH4PhysicsofBipolarTransistorsBaseCurrentBasecurrentconsistsoftwocomponents:1)Reverseinjectionofholesintotheemitterand2)recombinationofholeswithelectronscomingfromtheemitter.127CH4PhysicsofBipolarTransistorsEmitterCurrentApplyingKirchoff’scurrentlawtothetransistor,wecaneasilyfindtheemittercurrent.128CH4PhysicsofBipolarTransistorsSummaryofCurrents129CH4PhysicsofBipolarTransistorsBipolarTransistorLargeSignalModelAdiodeisplacedbetweenbaseandemitterandavoltagecontrolledcurrentsourceisplacedbetweenthecollectorandemitter.130CH4PhysicsofBipolarTransistorsExample:MaximumRL

AsRLincreases,Vxdropsandeventuallyforwardbiasesthecollector-basejunction.Thiswillforcethetransistoroutofforwardactiveregion.Therefore,thereexistsamaximumtolerablecollectorresistance.131CH4PhysicsofBipolarTransistorsCharacteristicsofBipolarTransistor132CH4PhysicsofBipolarTransistorsExample:IVCharacteristics133CH4PhysicsofBipolarTransistorsTransconductanceTransconductance,gmshowsameasureofh