2013哈尔滨世界农业博览会

Section8.1HeithemSouissiDinaMiqdadiMohammadButtAhmadElmardiniWyattSullivanFaresAlnajjarBetarolloff&AvalanchebreakdownHEITHEMSOUISSIDINAMIQDADIBetaRolloffβRolloffBeta=CurrentgainforaBJTBetaiscommonlyreferredtoasaconstant,althoughitvariesconsiderablydependingonthecollectorcurrent.βRolloff(con’t)Highcurrentbetarolloffiscausedbyhigh-levelinjection.Lowcurrentbetarolloffresultsfromrecombination,andshallowemittereffect.βRolloff(con’t)ThelateralPNPbetacurveisdifferentfromthatofanNPNforseveralreasons1.Duetoitslightdoping,PNPexhibitsloweremitterinjectionefficiencythatreducesitsbetapeak.2.TheflowofcarriersnearthesurfaceofthePNPincreasessurfacerecombination,causinglow-currentbetarolloff.3.ThelightlydopedbaseofthePNPcauseshighlevelinjectionstobeginatrelativelylowcurrentlevels,thereforehighcurrentbetarolloff.AvalancheBreakdownWhatisBreakdown?Deleteriouseffectthatoccursinthepresenceofhighelectricfield.Causeshighresistanceelementstoallowflowofhighcurrent.Typicallyanirreversibleeffectpermanentlydamagingtheelement.Avalanche/ZenerBreakdown'Zenerdiode'and'avalanchediode'aretermsoftenusedinterchangeably.Bothrefertobreakdownofadiodeunderreversebias.AvalancheoccurswithlightlydopedPNjunctions.(Multiplicationeffect).Zeneroccursinhighlydopedjunctions(quantumtunnelingeffect).Avalanche/ZenerBreakdown(con’t)Avalanche/ZenerBreakdown(con’t)Reversebias=Verylittlecurrentflow=OpencircuitAsReversevoltageapointisreachedwherecurrentdramatically,thereforedynamicresistance.AvalancheBreakdownAvalanchebreakdowncauseshighflowofcurrentunderreversebiascondition!Thequestionis:Howdoesthishappen?AvalancheBreakdown(con’t)ReversebiasThickdepletionregioncauseshighelectricfieldandtremendousaccelerationVeryfewelectronsmakeitthroughdepletionregionwithhighvelocityTheseelectronscollidewithatomsinthedepletionregionandfreemoreelectrons(ProcesscalledMultiplication).ResultsinhigherandhighercurrentflowAnempiricalrelationshipusedtodescribeavalanchebreakdown:M=Multiplicationfactor=1/(1-|V/Vbr|^n)AvalancheBreakdown(con’t)Byanalogy,theprocessisnamedbecauseasinglecarriercanspawnliterallythousandsofadditionalcarriersthroughcollisions,justasasinglesnowballcancauseanavalanche.AvalancheBreakdown(con’t)AvalancheBreakdown(con’t)ThermalRunaway,SecondaryBreakdownandSaturationinNPNTransistorsMohammadButtAhmadElmardiniTopicsinthissection(8.1.3-4)ThermalrunawayHotspotsSecondarybreakdownForward-biassafeoperatingarea(FBSOA)SaturationinNPNtransistorsCurrenthoggingPreventionschemesforcurrenthoggingThermalRunaway:Increaseintemperatureleadstohighercurrentandpowerdissipation,whichinturnincreasesthetemperaturefurtheruntilthedeviceisdestroyed.HotSpot:Theregionofthetransistorthatconductscurrentandsteadilyshrinksasitgrowshotteriscalledahotspot.Temp.increasesβincreasesICB0increasesVBEdecreasesUnstableHotSpot:Ifthetemperatureinthehotspotreaches350oCto450oC,thetransistorshortsout.StableHotSpot:Iftheincreasedcurrentdensitycausesbetatorollofffarenoughatahighbutnotdestructivetemperature,itstabilizesthehotspot.Atransistorwithstablehotspotself-destructsduringturn-offduetoavalancheofthecollector-basejunctionatavoltagebelowtheVCEOSecondaryBreakdown:HighvaluesofVCEandICcauseburnoutofatransistorjunctionarea.Itisnotnecessaryfortheaveragejunctiontemp.toexceedthemaximumrating.Secondarybreakdowncanalsooccurintransistorswhichhavenotexperiencedthermalrunaway.Thermalrunawayandsecondarybreakdowncanbeavoidedbyrestrictingtheoperatingconditionsofthetransistor.Forward-biasSafeOperatingArea(FBSOA)WhichtransistorloseslessFBSOAduetosecondbreakdown?Transistor1:Averyrobusttransistorwithoutaheatsink.Transistor2:Aproperlyheatsunkbutpoorlydesignedtransistor.•Answer:Transistor1SaturationinNPNTransistorsOccurswhenVBE0andVBC0UsefulinpowertransistorstoreduceVCE(sat)andtominimizepowerdissipationUnintentionalsaturationisthebiggestdevice-relateddesignflawAffectsdiscreteandintegratedtransistorsindifferentwaysIntegratedBipolarTransistorsCurrentHoggingCausedbysaturationduetoflowofbasecurrentthroughbase-collectorjunctionratherthanbaseemitterjunctionReducesthebase-emittervoltageBasecurrentofsaturatingtransistorincreasesattheexpenseofothertransistorsCurrentMirrorTransistorPreventionofCurrentHoggingBase-sideBallasting:Insertionofmatchedresistorsintobaseleadsofeachtransistor.Resistorsmustratioinverselytotheemitterareasoftheirrespectivetransistors.Schottkyclamps:Clampingdiodeisconnectedacrossbase-collectorjunctiontopreventsaturation.Base-sideBallastingSchottky-clampedTransistorLateralPNPTransistorsWyattSullivanFaresAlnajjarLateralParasiticTransistanceEmitter-P-substrate•Majorcauseofcollectorefficiency•NBLrequiredtocompensateCollector-BOI•Createssaturationcurrents•DeterminesSaturationvoltagesNBLExplainedN-typeBuriedLayer(NBL)Actsasaminoritycarrierrepellant•Holesare“repelled”backintotheN-epi•DriftcurrentscauseaslightpotentialSidewallTransistanceHighcollectorvoltagescreatePNP•Collector-base-sidewall(P-Type)•Narrowbaseinhighvoltages•Emittercontinuestoinjectcurrent•Excesscollectorcurrentflowst