BipolarJunctionTransistors.ppt

Chapter5BipolarJunctionTransistors ChapterGoals ExplorethephysicalstructureofbipolartransistorStudyterminalcharacteristicsofBJT Exploredifferencesbetweennpnandpnptransistors DeveloptheTransportModelforbipolardevices DefinefouroperationregionsoftheBJT Exploremodelsimplificationsfortheforwardactiveregion UnderstandtheoriginandmodelingoftheEarlyeffect PresentaPSPICEmodelforthebipolartransistor Discussbipolarcurrentsourcesandthecurrentmirror PhysicalStructure TheBJTconsistsof3alternatinglayersofn andp typesemiconductorcalledemitter E base B andcollector C Themajorityofcurrententerscollector crossesthebaseregionandexitsthroughtheemitter Asmallcurrentalsoentersthebaseterminal crossesthebase emitterjunctionandexitsthroughtheemitter Carriertransportintheactivebaseregiondirectlybeneaththeheavilydoped n emitterdominatesthei vcharacteristicsoftheBJT TransportModelforthenpnTransistor Thenarrowwidthofthebaseregioncausesacouplingbetweenthetwobacktobackpnjunctions Theemitterinjectselectronsintobaseregion almostallofthemtravelacrossnarrowbaseandareremovedbycollector Base emittervoltagevBEandbase collectorvoltagevBCdeterminethecurrentsinthetransistorandaresaidtobepositivewhentheyforward biastheirrespectivepnjunctions Theterminalcurrentsarethecollectorcurrent iC thebasecurrent iB andtheemittercurrent iE TheprimarydifferencebetweentheBJTandtheFETisthatiBissignificant whileiG 0 npnTransistor ForwardCharacteristics ForwardtransportcurrentisISissaturationcurrent VT kT q 0 025Vatroomtemperature Basecurrentisgivenby Emittercurrentisgivenby Inthisforwardactiveoperationregion npnTransistor ReverseCharacteristics Reversetransportcurrentis Emittercurrentisgivenby isreversecurrentgain Basecurrentisgivenby Basecurrentsinforwardandreversemodesaredifferentduetoasymmetricdopinglevelsintheemitterandcollectorregions npnTransistor CompleteTransportModelEquationsforAnyBias Thefirstterminboththeemitterandcollectorcurrentexpressionsgivesthecurrenttransportedcompletelyacrossthebaseregion Symmetryexistsbetweenbase emitterandbase collectorvoltagesinestablishingthedominantcurrentinthebipolartransistor pnpTransistor Operation ThevoltagesvEBandvCBarepositivewhentheyforwardbiastheirrespectivepnjunctions Collectorcurrentandbasecurrentexitthetransistorterminalsandemittercurrententersthedevice pnpTransistor ForwardCharacteristics Forwardtransportcurrentis Basecurrentisgivenby Emittercurrentisgivenby pnpTransistor ReverseCharacteristics Reversetransportcurrentis Basecurrentisgivenby Emittercurrentisgivenby pnpTransistor CompleteTransportModelEquationsforAnyBias CircuitRepresentationforTransportModels Inthenpntransistor expressionsanalogousforthepnptransistors totalcurrenttraversingthebaseismodeledbyacurrentsourcegivenby Diodecurrentscorresponddirectlytothe2componentsofbasecurrent OperationRegionsoftheBipolarTransistor Base emitterjunction Base collectorjunction i vCharacteristicsBipolarTransistor Common EmitterOutputCharacteristics ForiB 0 thetransistoriscutoff IfiB 0 iCalsoincreases ForvCE vBE thenpntransistorisintheforwardactiveregion iC bFiBisindependentofvCE ForvCE vBE thetransistorisinsaturation ForvCE 0 therolesofcollectorandemitterarereversed i vCharacteristicsofBipolarTransistor Common EmitterTransferCharacteristic Thischaracteristicdefinestherelationbetweencollectorcurrentandbase emittervoltageofthetransistor Itisalmostidenticaltothetransfercharacteristicofapnjunctiondiode SettingvBC 0inthecollector currentexpression JunctionBreakdownVoltages Ifreversevoltageacrosseitherofthetwopnjunctionsinthetransistoristoolarge thecorrespondingdiodewillbreakdown Theemitteristhemostheavilydopedregion andthecollectoristhemostlightlydopedregion Duetothesedopingdifferences thebase emitterdiodehasarelativelylowbreakdownvoltage 3to10V Thecollector basediodeistypicallydesignedtobreakdownatmuchlargervoltages Transistorsmustthereforebeselectedinaccordancewiththepossiblereversevoltagesincircuit SimplifiedForward ActiveRegionModel Intheforward activeregion thebase emitterjunctionisforward biasedandthebase collectorjunctionisreverse biased vBE 0 vBC 0 Ifweassumethatthenthetransportmodelterminalcurrentequationssimplifyto TheBJTisoftenconsideredacurrent controlledcurrentsource althoughfundamentalforwardactivebehaviorsuggestsavoltage controlledcurrentsource SimplifiedCircuitModelforForward ActiveRegion Currentinthebase emitterdiodeisamplifiedbythecommon emittercurrentgainbFandappearsatthecollectorThebaseandcollectorcurrentsareexponentiallyrelatedtothebase emittervoltage Thebase emitterdiodeisoftenreplacedbyaconstantvoltagedropmodel VBE 0 7V sinceitisforward biasedintheforward activeregion SimplifiedForward ActiveRegionModel AnalysisExample Problem FindQ pointGivendata bF 50 VBC VB VC 9VAssumptions Forward activeregionofoperation VBE 0 7VAnalysis BiasingforBJT ThegoalofbiasingistoestablishaknownQ point whichinturnestablishestheinitialoperatingregionoftransistor InBJTcircuits theQ pointisrepresentedby VCE IC forthenpntransistoror VEC IC forthepnptransistor Ingeneral duringcircuitanalysis weuseasimplifiedmathematicalrelationshipsderivedforthespecifiedoperationregionofthetransistor ThepracticalbiasingcircuitsusedwithBJTsare TheFour ResistorBiasnetworkTheTwo ResistorBiasnetwork Four ResistorBiasNetworkforBJT Q pointis 4 32V 201mA BELoop CELoop Four ResistorBiasNetworkforBJT CheckAnalysis Allcalculatedcurrents 0 VBC VBE VCE 0 7 4 32 3 62VHence thebase collectorjunctionisreverse biasedandtheassumptionofforward activeregionoperationiscorrect Theload lineforthecircuitis Thetwopointsneededtoplottheloadlineare 0 12V and 314mA 0 Theresultingloadlineisplottedonthecommon emitteroutputcharacteristicsforIB 2 7mA TheintersectionofthecorrespondingcharacteristicwiththeloadlinedeterminestheQ point Four ResistorBiasNetworkforBJT DesignObjectives FromtheBEloopanalysis weknowthatThiswillimplythatIB IB for Four ResistorBiasNetworkforBJT DesignGuidelines ChooseI2 IC 5 Thismeansthat R1 R2 5VCC IC LetICRC IERE VCC VCE 2 ThenRC VCC VCE 2IC RE aFRCIfREQ bF 1 RE thenIERE VEQ VBE Then VCC VCE 2 VEQ VBE orVEQ VCC VCE VBE 2 SinceVEQ VCCR1 R1 R2 and R1 R2 5VCC IC ThenR2 5VCC IC R1 CheckthatREQ bF 1 RE Ifnot adjustbullets1and2above Note IntheLabVIEWbiascircuitdesignVI NPNBias vi bullet1iscalledthe BaseMargin andbullet2iscalledthe C EV oltage Drops Problem5 874 RBiasCircuitDesign Two ResistorBiasNetworkforBJT Example Problem FindtheQ pointforthepnptransistorinthe2 resistorbiascircuitshownbelow Givendata bF 50 VCC 9VAssumptions Forward activeregionoperationwithVEB 0 7VAnalysis Q pointis 6 01mA 2 87V PNPTransistorSwitchCircuitDesign EmitterCurrentforPNPSwitchDesign BJTPSPICEModel Besidesthecapacitanceswhichareassociatedwiththephysicalstructure additionalmodelcomponentsare diodecurrentiS capacitanceCJS relatedtothelargeareapnjunctionthatisolatesthecollectorfromthesubstrateandonetransistorfromthenext RBistheresistancebetweenexternalbasecontactandintrinsicbaseregion CollectorcurrentmustpassthroughRConitswaytotheactiveregionofthecollector basejunction REmodelsanyextrinsicemitterresistanceinthedevice BJTPSPICEModel TypicalValues SaturationCurrent 3e 17AForwardcurrentgain 100Reversecurrentgain 0 5ForwardEarlyvoltage 75VBaseresistance 250WCollectorResistance 50WEmitterResistance 1WForwardtransittime 0 15nsReversetransittime 15ns MinorityCarrierTransportinBaseRegion Withanarrowbaseregion minoritycarrierdensitydecreaseslinearlyacrossthebase andtheSaturationCurrent NPN is whereNAB thedopingconcentrationinthebaseni2 theintrinsiccarrierconcentration 1010 cm3 nbo ni2 NABDn thediffusivity kT q mnSaturationcurrentforthePNPtransistoris Duetothehighermobility m ofelectronscomparedtoholes thenpntransistorconductshighercurrentthanthepnpforequivalentdopingandappliedvoltages DiffusionCapacitance ForvBEandhenceiCtochange chargestoredinthebaseregionmustalsochange Diffusioncapacitanceinparallelwiththeforward biasedbase emitterdiodeproducesagoodmodelforthechangeinchargewithvBE Sincetransportcurrentnormallyrepresentscollectorcurrentintheforward activeregion EarlyEffectandEarlyVoltage Asreverse biasacrossthecollector basejunctionincreases thewidthofthecollector basedepletionlayerincreasesandtheeffectivewidthofbasedecreases Thisiscalled base widthmodulation InapracticalBJT theoutputcharacteristicshaveapositiveslopeintheforward activeregion sothatcollectorcurrentisnotindependentofvCE Early effect WhentheoutputcharacteristicsareextrapolatedbacktowheretheiCcurvesintersectatcommonpoint vCE VA Earlyvoltage whichliesbetween15Vand150V SimplifiedF A R equations whichincludetheEarlyeffect are BJTCurrentMirror ThecollectorterminalofaBJTintheforward activeregionmimicsthebehaviorofacurrentsource OutputcurrentisindependentofVCCaslongasVCC 0 8V ThisputstheBJTintheforward activeregion sinceVBC 0 1V Q1andQ2areassumedtobea matched pairwithidenticalIS bFO andVA BJTCurrentMirror continued Withaninfi