45 NO 12 DECEMBER 1998 2437 Analysis of SiGe Heterojunction Integrated Injection Logic I L Structures Using a Stored Charge Model Simon P Wainwright Member IEEE Stephen Hall Member IEEE Peter Ashburn Member IEEE and Andrew C Lamb Abstract A ID: 25033
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IEEETRANSACTIONSONELECTRONDEVICES,VOL.45,NO.12,DECEMBER1998AnalysisofSi:GeHeterojunctionIntegratedInjectionLogic(I L)StructuresUsingaStoredChargeModelSimonP.Wainwright,Member,IEEE,StephenHall,Member,IEEE,PeterAshburn,Member,IEEE,andAndrewC.LambAbstractÐAquasi-two-dimensionalstoredchargemodelisdevelopedasanaidtotheoptimizationofSiGeintegratedinjectionlogic(IL)circuits.Themodelisstructure-basedandpartitionsthestoredchargebetweenthedifferentregionsoftheILgate.BoththeNpNswitchingtransistorandthePNploadtransistorarecorrectlymodeledandtheeffectsofseriesresistancesonthegateoperationaretakenintoaccount.Themodelisappliedtosurface-fedandsubstrate-fedvariantsofSiGeLandtheGeanddopingconcentrationsvariedtodeterminetheimportanttradeoffsinthegatedesign.Atlowinjectorcurrents,thesubstrate-fedvariantisfoundtobefasterbecauseoflowervaluesofcriticaldepletioncapacitances.Athighinjectorcurrents,theperformanceofbothvariantsislimitedbyseriesresistances,particularlyintheNpNemitterlayer.Theinclusionof16%Geinthesubstrate-fedILgateleadstoadecreaseinthedominantstoredchargebyafactorofmorethanten,whichsuggeststhatgatedelayswellbelow100psshouldbeachievableinSiGeIevenatageometryof3 N-typeSidopingconcentrationinthesubstrate(usedexclusivelyinC-I L). N-typeSidopingconcentrationintheswitchemit-ter(usedexclusivelyinC-I L). N-typeSidopingconcentrationintheinjectorbase(usedexclusivelyinC-I L). P-typeSidopingconcentrationintheinjectoremit-ter(usedinbothC-I LandSF-I L). p-typeSiGedopingconcentrationintheinjectorcollector/switchbase(usedinbothC-I LandSF- L). N-typeSidopingconcentrationintheswitchcol-lector(usedinbothC-I LandSF-I L). N-typeSidopingconcentrationintheinjectorbase/switchemitter(usedexclusivelyinSF-I ManuscriptreceivedJanuary29,1998.ThereviewofthispaperwasarrangedbyEditorW.Weber.ThisworkwassupportedbytheEngineeringandPhysicalSciencesResearchCouncil.S.P.WainwrightwaswiththeDepartmentofElectricalEngineeringandElectronics,UniversityofLiverpool,LiverpoolL693BX,U.K.HeisnowwithFagorElectronica,Mondragon,Guipuzcoa,Spain(e-mail:swain- Widthoftheswitchemitter(usedexclusivelyin L). Widthoftheinjectorbase(usedexclusivelyin L). Widthoftheinjectorcollector/switchbase(usedinbothC-I LandSF-I L). Widthoftheswitchcollector(usedinbothC-I andSF-I L). Widthoftheinjectorbase/switchemitter(usedexclusivelyinSF-I L). Depthoftheintrinsicstructures(intothepage). Depthoftheextrinsicbaserails(intothepage). UpwardcurrentgainoftheN-p-Nswitchtransistor. UpwardcurrentgainoftheP-N-pinjectortransis- N-p-N(switch)Gummel-Poonforwardtransportcoefcient. P-N-p(injector)Gummel-Poonforwardtransportcoefcient. Forwardcommonbasecurrentgainofswitch. Forwardcommonbasecurrentgainofinjector. Reversecommonbasecurrentgainofswitch. Reversecommonbasecurrentgainofinjector. LParameters Lengthofinjectorbase/switchemittercontact. Spacingbetweeninjectorbase/switchemittercon-tactandedgeofswitchbase/injectorcollector Lengthofswitchbase/injectorcollectorimplanta- Lengthofswitchcollectorcontact. Spacingbetweenswitchcollectorcontactandedgeofswitchbase/injectorcollectorimplantation. Lengthofswitchbase/injectorcollectorcontact. Lengthofswitchcollector . Lengthofactivearea . Lengthoftotalarea . Electronstoredchargeinthesubstrate.(injector1998IEEE IEEETRANSACTIONSONELECTRONDEVICES,VOL.45,NO.12,DECEMBER1998 Holestoredchargeintheinjectorbase/switch Electronstoredchargeintheswitchbase/injector Holestoredchargeintheswitchcollector. Holestoredchargeintheswitchcollectorinjectedfromtheextrinsicbaserails. Electronstoredchargeintheextrinsicbaserails. Switchemitter/base(injectorbase/collector)deple-tioncharge. Switchcollector/basedepletioncharge. Switchcollector/basesidewalldepletion.chargeConventional(Surface-Fed) LParameters Lengthofswitchemittercontactandisolationtoedgeofswitchbase/injectorcollectorimplantion. Lengthofswitchbase/injectorcollectorimplan- Lengthofinjectorbaseregion. Lengthofisolationimplantationbetweeninjectorbaseandswitchcollector. Lengthofswitchcollectorregion. Lengthofswitchbase/injectorcollectorimplan- Lengthofswitchbase/injectorcollectorregion . Lengthofactivearea . Lengthoftotalarea . Holestoredchargeinthesubstrate. Holestoredchargeintheswitchemitter. Electronstoredchargeintheswitchbase/injector Holestoredchargeintheinjectorbase. Holestoredchargeintheswitchcollector. Holestoredchargeintheswitchcollectorin-jectedfromtheextrinsicbaserails. Holestoredchargeintheinjectorbaseinjectedfromtheextrinsicbaserails. Electronstoredchargeintheextrinsicbaserails. Switchemitter/basedepletioncharge. Switchcollector/basedepletioncharge. Injectorcollector/basedepletioncharge. Injectorcollector/basesidewalldepletioncharge. Switchcollector/basesidewalldepletioncharge.I.IIGH-performancebipolarlogiccircuitsareusuallyreal-izedusingemittercoupledlogic(ECL)butthattechnol-ogyfeaturesrelativelylowpackingdensityandhighpowerdissipation.IntegratedinjectionlogicorI L[1],[2]isalowpowerbipolartechnologysuitableforVLSIwhichtraditionallyhassufferedfromarelativelypoordynamicperformance.TheminimumgatedelayofSiI L[3]isprimarilydeterminedbystoredchargeinparasiticdiodesassociatedwiththeextrin-sicbaseregionsoftheI Lgate.Self-alignedcollector-basestructureshavebeenreported[4]whichminimizetheareaoftheseparasiticdiodesanddeliveragatedelayof0.8nsforalayoutgeometryof2.5 mandafan-outof3.Otherparasiticsthatcanin uencethegatedelayareseriesresistanceeffectsinthebaseandchargestorageintheintrinsicbaseofthepnpRecentlytherehasbeenrenewedinterestinI L,motivatedbytheimpressiveperformancereportedforSiGeheterojunc-tionbipolartransistors.InthecontextofI L,SiGetechnologyofferstheprospectofusingbandgapengineeringtominimizethestoredchargeintheparasiticdiodesassociatedwiththe Lgate.ModelingofSiGeI Lhasbeenreportedrecently[5],[6]butwithconsiderablecompromiseinthetreatmentofthepnploadstructureandhencetheomissionofimportantparasiticelementsinthemodels.TheworkofMazhari.[5]representedtheloadasacurrentsource,implementedbyavoltagesourceandresistor,whereasKarlsteenetal[6]representedtheloadwithanidealcurrentsource.Theseapproximationsthereforeprecludeconsiderationofthechargestorageassociatedwiththeloadandalsopredictincorrectlythecurrentdeliveredbytheload;thelatterbecausetheloadtransistoriseitherintheactiveorsaturatedregime,dependingontheprevailinglogiccondition.Moreover,theseapproachesdonotconsidertheinherenttradeoffsinthedesignofthemergedn-p-n/p-n-pdevices,whichplacesevereconstraintsontherelativedopinglevelsofthesemiconductorregionsandtheGeconcentrationofthen-p-nbaselayer,asoutlinedinSectionII.Furthermoretheworkpresentedin[5]onlyconsidersthegatedelayofcircuitswhich,althoughbeingakeypartinthetotaldelay,neglectscomponentsofcapacitanceandlateralseries`access'resistances.TheapproachusedhereisbasedonthemodelofHen-dricksonandHuang[7]andinvolvesdividingthestructureintodiscreteregionsforeachofwhichthechargeinjectionconditionscanbecalculated.Thusthestoredchargecanbeascertainedthroughoutthestructureandforagiveninjectorcurrent,thepropagationdelayoftheinvertercanbecal-culated.Themodelisextendedfromthatdescribedin[7]inthatjunctionvoltagesarecalculatedforagiveninjectorcurrentandloadgate.Thusallterminalcurrentsareknownandaquasi-two-dimensionalapproachisthenemployedtocalculateterminalvoltagesusingsimplespreadingresistancescalculatedfromthesemiconductorlayerparameters,assumingthedepletionapproximation.Themodelisthereforequasi-two-dimensional.WehavechosentoapplythemodeltothreespecicSiGeI Ldesigns,oneaconventionalsurface-fedapproach,thesecondsubstrate-fed[8]andthethirdavariantwhichisspecicallyoptimizedforSiGeI Landfeaturesahighdegreeofself-alignment.Thelatterservestodemonstratethe exibilityofthemethodandalsopredictsagatedelayof34ps.ThustheuseofheterojunctionscanaddhighspeedtotheotherwellknownadvantagesofI Ltechnology,namelyhighpackingdensity,lowvoltageandlowpowerdissipation.II.I ATEAschematicdiagramofanI LinverterisshowninFig.1.WeareconcernedherewithtwoformsofI L:substratefedlogic(SF-I L)andconventionalsurfacefedlogic(C-I L)of WAINWRIGHTetal.:ANALYSISOFSi:GeHETEROJUNCTIONINTEGRATEDINJECTIONLOGIC2439 Fig.1.Circuitdiagramofanintegratedinjectionlogicinverter. Fig.2.Schematiccross-sectionalviewofthesubstrate-fedintegratedinjec-tionlogic(SF-IL)gate.Dimensions(m)are,lateral:=10=15.Intrinsicdevicewidth,=12andwidthofextrinsicbaserails,whichthesimpliedcrosssectionsareshowninFigs.2and3.ThesestructureshavebeendesignedtobeconsistentwiththeepitaxialbaseandcollectorprocessesthatareneededforaSiGetechnology.TheoriginalconventionalsurfacefedI Lcircuitsreportedin[1],[2]featuredalateralP-N-Pinjectortransistorwhichwasinefcientindeliveringthebasecurrentfortheverticalswitchingtransistor.ThelateralP-N-Palsoledtothepoordynamicperformanceoftheoriginal Lcircuitsasaresultofexcessivechargestorageinthevicinityoftheemitteroftheswitchingtransistorwhichismergedwiththebaseoftheinjectortransistor.TheSiGeversionofC-I LshowninFig.3hasaverticalP-N-p(upper-casedenotesSi,lower-caseSiGe),whichhasthepotentialtoovercometheproblemofpoordynamicperformancebecausetheheterojunctioncanlimittheholeinjectionbackintothebaseoftheP-N-pinjectortransistorandalsointotheN-substrate.TheSiGecollectoroftheinjectortransistorismergedwiththebaseoftheN-p-NswitchingtransistorandtheN-substrateformstheemitteroftheswitchingtransistor.IntheSF-I Lvariant[8],showninFig.2,theemitteroftheinjectortransistorisassignedtothesubstrate,therebyensuringanefcientsupplyofbasecurrentfortheswitchingtransistor. Fig.3.Schematiccross-sectionalviewofthesurface-fedintegratedinjectionlogic(C-IL)gate.Dimensions(m)are,lateral:=15=10=30=10=10.Intrinsicdevicewidth,=15widthofextrinsicbaserails,Clearly,thesubstratefedversionhashigherpackingdensityandinherentlylowercapacitancethanthesurfacefed,butattheexpenseofmorecomplexdevicedesigntradeoffs.TheinherentadvantagesofthedesignsshowninFigs.2and3arerstlythattheheterojunctionemitter/basestructureoftheN-p-Nswitchingtransistorproduceshigh forimproveddynamicfan-out.Secondlytheheterojunctioncollector/basestructurefortheinjectortransistorreducesholeinjectionintothebaseoftheinjector(alsotheemitteroftheswitchinSF-I L)reducingchargestorageinthatregion.Thirdlymodernlowtemperaturegrowthtechniques(MBEorLPCVD)ensureexcellentcontrolofepitaxiallayerthicknesses.ThisisespeciallyimportantfortheSF-I LN-p-Nemitterlayerwhichhasastrongin uenceondynamicperformanceandwasamajordrawbackoftheoriginaltechnology.Finally,epitaxialSiGebasesaremorereadilyscalable,whichisanimportantadvantageoverSiI A.DesignConstraintsAnanalyticalmodeloftheI LinverterhasbeenpresentedbyKlaassen[9]whichallowstheconditionsforachievinginverteractiontobedened.ReferringtoFig.1,physically,therequirementisthatthemergedP-N-p(injector)andN-p-N(switch)transistorsshouldbedesignedsuchthatthevoltageassociatedwithalogic1onthebaseoftheN-p-N besufcientlylowerthanthepowerrailvoltage, ,sothattheP-N-pinjectorisabletosupplysufcientcurrent,thatistosay,theP-N-pmustnottotallysaturate.Thereisalsoalimittotheªupwardºgain, ,oftheN-p-Nswitchtransistor.Thesetwoconditionscanbesummarizedas (1) (2)where istheGummel-Poonforwardsaturationcurrent,thesubscriptsªNºandªPºrefertotheN-p-NandP-N-pdevicesrespectivelyand (3) IEEETRANSACTIONSONELECTRONDEVICES,VOL.45,NO.12,DECEMBER1998 electroniccharge; emitter-basejunctionarea; diffusioncoefcientofminoritycarriersinthebase; intrinsiccarrierconcentration,dependentonboththeGeconcentrationandband-gapnarrowingintheheav-ilydopedbase; basewidth; dopingconcentrationofthebase,whichisassumedReferringto(3),wenowlisttheimportantdesigntradeoffsassociatedwiththeratioof whichguresin(1)and1)IncreasingtheGeconcentrationincreases duetoareductioninbandgapandhastheaddedadvantageofdeferringthehighinjectionconditioninthebaseoftheN-p-Nswitchingtransistortoahigherbiasvalue.HowevertheexcesschargestorageinthebaseoftheN-p-Nswitchingtransistorisincreasedwhichaffectsthedynamicperformance.2)ReducingtheSiGedopingcausesanetincreasein ,asthebandgapnarrowingandotherdependenciesareweaker,in(3)thanthatof .Reducing howeverhastheadverseeffectsofincreasingtheseriesresistanceandchargestorageinthebaseoftheN-p-Nswitchingtransistoraswellasreducingthebiaslevelfortheonsetofhighinjection.3)Increasing (P-N- basewidth)directlydecreases butalsoadverselyincreasesthechargestorageintheinjectorbase.Alarger hastheadvantageofreducingtheseriesresistanceinthebaseoftheP-N-pandalso,fortheSFvariant,intheemitteroftheN-p-Ndevice.Thisisanimportantadvantage,asthecurrentisthislayerwillbehighwithalogic1attheinput.4)Increasingtheinjectorbasedopingreduces decreasesseriesresistanceandincreasesthebiaslevelfortheonsetofhighlevelinjection.ThereisalsoadisadvantagefortheSF-I Lvariant,asitalsoincreasesthebase-emittercapacitancefortheswitchingtransistorwhichlimitsswitchingspeeds.TheseimportantdesigntradeoffsplacesevereconstraintsontheGeconcentrationanddopinglevelsbutarevitaltotakeintoaccountinarealisticappraisalofI L.Inparticular,spreadingresistance,especiallyintheN-p-Nemitterlayercanpreventinverteraction[10].FurthercommentisdeferredtothediscussioninSectionV.III.TInthissectionandtheassociatedappendix,wedescribethemodelwhichallowscalculationoftheaveragetimetoswitchchargebetweentwologiclevels,asafunctionofinjectorcurrent.Thedeviceisdividedintodiscretechargestorageregionsassociatedwithquasineutralanddepletedregions.ThedetailedexpressionsforthechargestorageineachregionconsideredarepresentedintheappendixandcanbeunderstoodbyreferringtothelistofsymbolsandtheregionaldenitionsinFigs.2and3.Theprincipleisdemonstratedbyconsiderationofthefollowingtwoequationswhichdescribetheswitched(free)chargeassociatedwithaquasi-neutralregionboundedbyinjectingandcollectingjunctions: andthatfortheswitchedchargeassociatedwithadepletion (5)where electroniccharge; area; intrinsiccarrierconcentrationofthesemiconductor; dopingconcentrationoftheregion; thicknessoftheepitaxiallayer(orthediffusionlengthifnecessary); appliedjunctionvoltage; built-injunctionvoltage; thermalvoltage.Thevoltagetermsinexpressions(4)and(5)accountforthedifferenceinjunctionvoltageatlogicª1ºandlogicª0ºandaredenedas: (6) wherethesubscriptsª1ºandª0ºrelatetohighandlowlogicconditionsrespectively.Theaveragetimetoswitchbetweenthetwologiclevelsasafunctionofinjectorcurrentisfound AlsoincludedinthemodelarethespreadingresistancesofeachregionwhichwhencombinedwiththeEbers-Mollmodelscanbeusedtocalculatetheterminalvoltagesasfollows.Theoperatingconditions ,foragiveninjectorcurrent,aredeterminedbythefollowingiter-ativeprocedure.First,thecommonemitter andcommon currentgainsandthespreadingresistancesofthedifferentlayers,arecalculatedfromthedopinglevelsandthicknessesofthevariousregions,ignoringdepletionregions.Bothreverseinjectionandneutralbaserecombinationareconsideredasmechanismsforbasecurrent.Dopingdependentdiffusioncoefcientsandband-gapnarrowingaretakenintoaccountusingtheempiricalexpressionsincludedin[11]and[12].TheeffectofGeconcentrationontheintrinsiccarrierconcentrationistakenfrom[13].Junctionvoltagesandcur-rentsarethencalculatedusingtheEbers-Molltransportmodeltogivetheemitter-basevoltageoftheswitchtransistor,as (9) WAINWRIGHTetal.:ANALYSISOFSi:GeHETEROJUNCTIONINTEGRATEDINJECTIONLOGIC2441andthebase-collectorvoltageoftheinjectortransistoras: ReferringtoFig.2,theemittercurrentfortheinjectortransis-toroftheSF-I Lversiononlyis Thefactorª ºin(11)isanestimateofthefractionoftheemittercurrentthatiscollectedinthebaseoftheswitchtransistor.ThecollectorcurrentsoftheN-p-NswitchtransistorofthegateunderconsiderationandtheP-N-pinjectortransistorofthefollowinginverterareequal ,andsowecanwrite Theactivevaluefor isusedin(12)becausetheinjectortransistoroftheloadinggateisunsaturatedwithalogiczeroontheoutputofthetestgate.Wecanalsowrite Now,recognisingthat ,(9)and(10)areequatedandrearrangedtoallowcalculationof therstinverterandhence canbefound.Thejunction canalsobefoundfromtheappropriateequationoftheformof(10).Allcurrentscanthusbedeterminedintheªintrinsicºinverterandhencethevoltagedropacrosstheswitchemitterlayerwithseriesresistance, ,canbecalculatedas fortheSF-I Lstructure fortheC-I Lstructure Similarlythevoltagedropacrossthecollectorseriesresistance, ,canbecalculatedusingthefollowingexpressions: Thisenablesthevoltagestobefound.Thesevaluesforterminalvoltagearethenusedtogiveªworstcaseºestimatesofdepletionregionwidthsanddependentparam-etersandtheterminalvoltagesarere-calculatedusingad-justedvaluesofquasineutralregions.Finallythevariouschargecomponents arecalculatedusing(A1)±(A13)or(A14)±(A28)andthepropagationdelay ,fortheinverterswitchingbetweenthetwologiclevels,foragiveninjectorcurrentiscalculatedfrom(8).Theinjectorrailvoltagecanbecalculatedfrom (16)where istheseriesresistanceassociatedwiththeemitteroftheinjectortransistor.Thevalueofinjectorvoltageob-tainedfrom(16)allowsthecalculationoftheaveragepower Fig.4.ValidationofthechargestoragemodelbyacomparisonofthepredictedswitchingtimeasafunctionofinjectioncurrentwiththemeasuredresultsofTangetal.[4].IV.RESULTSA.ValidationoftheModelingTechniqueTheself-alignedSiI LstructureofTangetal.[4]wasimplementedinthemodeltoprovideexperimentalvalidationoftheanalysismethod.Averagevaluesforimpuritylevelsofdiffusedregionswereused,deducedfromthesheetresistancevaluesgivenandtakingintoaccountdopingdependenceofmobilitybasedontheaveragevalues.Thereported,measuredvaluesfortransistorcurrentgains wereused.TheresultsareshowninFig.4anddespitetheapproximationsmade,verygoodagreementisobtainedbetweentheexperimentaldatapointsandthesimulatedswitchingtime,whichprovidescondenceinthemodelingtechniqueusedhereforpredictivesimulations.Itisworthnotingthattheassumptionofconstant,averagedopingconcentrationismoreappropriateforthecaseoftheepitaxialSiGestructuresconsiderednext.B.SiGeI LSimulationsTherearetwoaspectsofI Loperationthatmustbeconsid-ered;rstlythefactthatdimensionsanddopingconcentrationsshouldsatisfytheconditionsforinvertingaction,andsecondly,withinthoserestrictions,thattherelativeparametersshouldbeoptimizedtoobtainthebestpossibletransientperformance.Theparametersneededtosatisfytheinvertingactioncondi-tionsweredeterminedbytwo-dimensional(2-D)simulation(MEDICI)fromwhichtheSPICEdcmodelparameterswereextracted.Theresultsofthisprocess[10]identiedtheim-portanceofminimizingspreadingresistance,particularlyintheswitchemitter,inadditiontotherequirementssetby(2)and(3).ThematerialparametersthatarerequiredtoallowinvertingactionaregiveninTableI.UsingthedopingconcentrationsinTableIitwasfoundthata16%molefractionofGewasrequiredtoensureinvertingaction.Thevaluesforswitchemitterdopingconcentrations, and ,needtobehighinordertoreducespreadingresistances.Usingthesepreliminaryresultsthechargestoragemodelcanbeutilizedtodeterminetheswitchingdelaytime,thepower- IEEETRANSACTIONSONELECTRONDEVICES,VOL.45,NO.12,DECEMBER1998 (a)(b)Fig.5.Calculatedcomponentsofstoredchargeasafunctionofasafunctionofinjectioncurrent:(a)substrate-fedIntegratedInjectionLogic(SF-and(b)surface-fedintegratedinjectionlogic(C-ITABLEIUMMARYOFTHEATERIALANDSEDINTHEALCULATIONS delaycharacteristicsandtoidentifywhichchargestorageregionlimitsperformance.Wepresentresultsfor2.5 designrules,unlessotherwisestated,inordertoalloweasycomparisonwiththeliterature.Fig.5showsthemagnitudeofthechargecomponentsasafunctionofinjectorcurrent, ,foreachstructureusingtheparametersinTableI.Itcanbeseenthatinbothsubstratefed[Fig.5(a)]andsurfacefed[Fig.5(b)]variantsthemostsignicantelementsofchargearethoseassociatedwiththedepletionregions(the Toreducethesechargeelementsthestructuraldimensionsand/orthedopingconcentrationsoftheseregionshavetobereduced.Unfortunatelythislattermodicationwillcauseanincreaseincriticalseriesresistancesintheassociatedregionsandasaresultthereistradeoffbetweendepletioncapacitanceandseriesresistance,aswillbediscussedlater.TheeffectofintroducingSiGeintothebaseoftheswitchtransistorisillustratedusingthespecicexampleofthetwolargestexcesschargetermsinthesubstratefedstructure, (holesstoredintheswitchemitter)and (electronsstoredintheswitchbase).Theresultsareshownin Fig.6.Storedholechargeintheswitchemitterandstoredelectronchargeintheswitchbaseasafunctionofinjectioncurrentforsubstrate-fedintegratedinjectionlogicgates(SF-IL)with0%Geand16%Fig.6foraSiGe(16%Ge)baseandapureSibase(0%Ge).ItisobservedthattheinclusionofSiGe(16%Ge)inthebaseoftheswitchtransistoreffectivelyreduces,foragivencurrentlevel,thereverseinjectionofholesfromthebasetotheemitter bymorethananorderofmagnitudebyvirtueoftheheterojunctionaction.AdisadvantageofintroducingSiGeistheincreaseinelectronstorageintheswitchbase totheincreaseinintrinsiccarrierconcentrationinthereducedbandgapSiGebase.ThebenetsofSiGeclearlyovercomethedisadvantagesinastructurewithadequatedimensions,astheincreaseinelectronstorageinthebaseislittlemorethanafactorofthree.Thedifferenceinsize,dopinglevelsandspreadingresistancesbetweenthetwostructuresexplainsthefactthatthisstoredelectronchargeisgreaterintheC-I LthanintheSF-I L.TheseresultsimplythataproperlyoptimizedSiGeI LtechnologycouldpotentiallydeliveraswitchingtimethatwasmorethantentimesfasterthanitsSiequivalent. WAINWRIGHTetal.:ANALYSISOFSi:GeHETEROJUNCTIONINTEGRATEDINJECTIONLOGIC2443 Fig.7.Calculatedswitchingtimeasafunctionofinjectioncurrentforthesubstrate-fed(SF-IL)andsurface-fed(C-IL)integratedinjectionlogicgates. Fig.8.Calculatedpower-delayproductasafunctionofinjectioncurrentforthesubstrate-fed(SF-IL)andsurface-fed(C-IL)IntegratedInjectionLogicTheswitchingdelaytimes,powerdelayproductandin-trinsicandterminalvoltage(logic)levelscanbeseeninFigs.7±9,respectively.ThelinearnatureofthedelaytimecurveobservedinFig.7conrmsthattheswitchingattheseinjectioncurrentlevelsislimitedbydepletioncharge.Thepowerdelay-productofSF-I Lcanbeseen,inFig.8,tobeapproximatelyafactorofthreelowerthanthatofC-I whichisconsistentwiththeswitchingdelaytimesshowninFig.7.ThepredictedlogiclevelsofthesubstratefedversionareshownasafunctionofinjectioncurrentinFig.9.Theterminalvoltagelevelscanbeseentodepartfromtheintrinsicpotentialsathigherinjectioncurrentsduetoparasiticpotentialdropsacrosstheseriesresistances.ThemostimportantcomponentofseriesresistanceisthatassociatedwiththeNepitaxiallayerthatformstheemitteroftheswitch(NpN)andthebaseoftheinjector(PNp).ThemagnitudeofthisaccessresistancecausesdebiasingoftheNpNtransistorathighcurrentlevelswhicheventuallyprohibitscircuitaction.Thecessationofcircuitfunctionalitycanbeseeninthiscasetooccurataninjectioncurrentof1mA. Fig.9.Predictedlogiclevelasafunctionofinjectioncurrentforthesubstrate-fed(SF-IL)integratedinjectionlogicgate.Alsoshownaretheintrinsiclogiclevelswhicharethelogiclevelsattheterminalsoftheintrinsicdevice(i.e.logiclevelsintheabsenceofseriesresistances). Fig.10.Calculatedswitchingtimeasafunctionofinjectioncurrentforasubstrate-fed(SF-IL)andasurface-fed(C-IL)integratedinjectionlogicgatedesignedwithagatedesignruleof1um.Alsoshownforcomparisonarethegatesthatweredesignedwithadesignruleof2.5um.V.DTheresultsinFigs.5±9indicatethatanumberoftradeoffsareinvolvedinthedesignofSiGeI Lgates.Wewillbeginbyconsideringthetradeoffsatlowinjectioncurrentswherethechargesinthedepletionregions dominatethegatebehavior.Itisclearthattoimprovetheswitchingtimeatlowinjectioncurrents,thedominantdepletionchargeshouldbereduced.FortheSF-I Lstructure,theswitchcollectordopingconsiderationshouldbedecreaseduntiltheswitchcollector/basedepletioncharge islowerthantheswitchemitter/basedepletioncharge [Fig.5(a)].Itisnotpossibletoreducetheswitchemitterdopinginordertoreduce ,becauseoflimitationsimposedbyseriesresistanceintheswitchemitter.FortheC-I LstructureinFig.5(b),thechargesthatneedtobeminimizedarethechargeintheinjectorcollector/basedepletionregion andintheswitchcollector/basedepletionregion, .Thiscanbeachievedbyreducingthedopinginthecollectorsofthe IEEETRANSACTIONSONELECTRONDEVICES,VOL.45,NO.12,DECEMBER1998 Fig.11.Schematiccross-sectionalviewofthesecondgenerationsurface-fedintegratedinjectionlogicgate.Layerwidths(m)are,nepi-layer,selectiveepi(inoxidewindows)045poly-silicon0.195,selectiveepi.ComponentsDEParespecictothisstructurebutarecalculatedusingthesameformofequations. Fig.12.Calculatedcomponentsofstoredchargeasafunctionofinjectorcurrentfortheself-alignedstructureofFig.11.injectorandswitchtransistors.Inthecaseoftheformer,thereisatradeoffwithseriesresistance,becausethecollectoroftheinjectortransistorisalsothebaseoftheswitchtransistor.Onceagain,itisnotdesirabletoreducetheswitchemitterdopingconcentrationduetolimitationsimposedbyseriesresistance.Theaboveconsiderationssuggestthat,atlowinjectioncurrentsthedirectbenetsofSiGearemarginalduetothedominanceofthedepletioncharge.Asmallbenetisobtained,whichismainlyduetothedecreasedchargeintheswitchemitter/baseandcollector/basedepletionregions.Atlowinjectioncurrents,themosteffectivewayofreducingthepropagationdelayistoreducetheareaofthedepletionregions,eitherbyreducingthegategeometryorbyemployingself-alignedfabricationschemestoreducetheextrinsicareasofthegate.Inscalingthedevicegeometry,SiGeislikelytobeofindirectbenet,sincetheincreasedgainoftheSiGeswitchtransistorwouldallowscalingtosmallergeometriesthancouldbeachievedusingapureSiI Ltechnology.Fig.10illustratestheeffectsofscalingthegategeometryontheswitching Fig.13.Calculatedswitchingtimeasafunctionofinjectorcurrentfortheself-alignedstructureofFig.11.time.Areductionofthegatedesignrulesfrom2.5 mto1.0leadstoadecreaseintheswitchingtimebyafactorofapproximately6forbothC-I LandSF-I L.ThebenetsofreducingtheextrinsicareasofthegatecanbeseeninthedifferenceinperformanceofC-I LandSF-I Lgates.TheoverallareaoftheSF-I Lgateissmallerthanthatofthe Lgatebecauseoftheuseofaninjectorinthesubstrate(Fig.2).Asaresult,thecriticaldepletioncapacitorsinthe LgatearesmallerthanthoseintheC-I Lgate.Evenfurtherbenetswouldbeobtainedifself-alignedfabricationschemeswereusedtoreducedepletionchargeandeliminatetheexcesschargeassociatedwiththeextrinsicbaserails[4].Suchastrategyhasachievedaswitchingtimeof290psinapureSiI Lgate[14]atagategeometryof3 mandoneforSiGeI Lispresentedinthenextsection.VI.ASSiGeI WehaveseenthatthebiggestbenetsofSiGearetobefoundathighinjectioncurrentswherethestoredcharges and dominatetheswitchingtime.Minoritycarrierstored WAINWRIGHTetal.:ANALYSISOFSi:GeHETEROJUNCTIONINTEGRATEDINJECTIONLOGIC2445chargeincreaseswithinjectioncurrent(Fig.5)andhencedominatesthegateswitchingtimeatthehighspeedendofthespeed-powercharacteristic.Anotherfactorwhichhastobetakenintoaccountinthisregionofoperationisseriesresistances,whichlimittheachievableswitchingspeeds.Acarefuloptimizationofthegatelayoutandarchitectureisneededtooptimizetheswitchingspeed,andthisisaddressedinthestructureofFig.11.Thestructurefeaturesaself-alignedSiGeHBTwhichminimizestheareaoftheextrinsicbaserails,andalateralpnpinjectorforcompatibilitywithmainstreamSiGetechnology.Seriesresistancesareminimizedbyincludingann buriedlayerwithasheetresistanceof20 sq.andap polysiliconextrinsicbasewhichissilicidedwithasheetresistanceof2 /sq.ThechargecomponentsfromapplicationofthestoredchargemodelareshowninFig.12anddelaycharacteristicinFig.13,whereamaximumdelayof34psispredictedusing1.4microndesignrules.Incalculatingthesecurves,aGeconcentrationof16%wasusedandbase,emitterandcollectordopingconcentrationsof4 10 cm 3 10 cm and1 10 cm OptimumperformanceatthehighestachievablecurrentlevelwasachievedbyensuringthatthedepletionandstoredchargecomponentsinthevicinityoftheNpNbaseareequal.Thepredictedgatedelayof34psis8.5timeslowerthanthereportedexperimentalvalueof290psfor3micronpureSiI gates[14],whichclearlydemonstratesthepotentialofSiGe L.Inaddition,thereisundoubtedlyscopeforimprovingonthegatedelaybyscalingthedevicegeometryandfurtheroptimisingthegatelayout.VII.CThepaperhaspresentedamodiedchargestoragemodelforuseintheinvestigationanddesignofSiGeI Lgates,takingaccountofthedetailedarchitectureofthegate.ThemodiedchargestoragemodelallowsidenticationofthedominantchargestorageregionsintheI Lgateandrepresentsapowerfulaidinoptimization.Themodelisstructure-based,includesbothswitchandloaddevicesandallowsforappro-priateloadingofinputandoutputofagiveninverter.Theimportanceofd.c.designconstraintshasbeenemphasisedsothatrealisticvaluesforparametersareused.Furthermore,theeffectsofseriesresistanceswhichprecludeoperationtohigherinjectorcurrents,isinherentinthemodelandisshowntobeaveryimportantaspectofI Lgatedesign.Atlowinjectorcurrents,theuseofSiGehasbeenshowntoofferonlymarginalbenets,sincetheswitchingspeedisdominatedbydepletionregioncharge.ThemostimportantadvantageofSiGeatthesecurrentlevelsislikelytobeim-provedscalabilityofI Ltechnology.Athighinjectorcurrents,wheretheswitchingspeedisdominatedbystoredminoritycarriercharge,theuseofSiGeinI Ltechnologyhasbeenshowntohaveimportantbenets.Areductionbyafactorofmorethanteninthestoredchargeisobtainedwhen16%Geisincorporatedintothebaseofthenpnswitchresistor.Themodelhasbeenappliedtoaself-alignedstructurewhichisspecicallyoptimizedforSiGeI Landaswitchingspeedof34psispredictedevenatageometryof1.4micron.Thisdelaywillbefurtherreducedwithafullyoptimized,scaleddesign.TORAGEQUATIONSFollowingtheapproachin[7],one-dimensional(1-D)ex-pressionscanbewrittenfortheexcessstoredchargeswitchedbetweenthetwologiclevelssetbytheinjectorcurrent,ineachoftheindividualregions.ReferenceshouldbemadetothelistofsymbolsandtheregionaldenitionsinFigs.2and3.SUBSTRATEFEDI Electronchargestoredinthesubstrate(injector whichconsistsofexcesselectronsintheinjectorHolechargestoredintheinjectorbase/switch whichconsistsofholesfromtheinjectoremitter(substrate)andtheswitchbase(SiGe).Thelatterissuppressedbytheheterojunctionaction.Electronchargestoredintheswitchbase/injector whichconsistsofelectronsfromtheswitchemitter(injectorbase)andswitchcollectorwhensaturated.Holechargestoredintheintrinsicswitchcollector whichconsistsofholesfromtheSiGebasewhentheswitchissaturated.Thiscomponentofstoredchargeissuppressedbytheheterojunctionaction.Holechargestoredintheswitchcollectorinjectedfromtheextrinsicbaserails whichconsistsofholesinjectedfromtheextrinsicbaserailsintotheswitchcollector.Electronsintheextrinsicbaserails (A6) IEEETRANSACTIONSONELECTRONDEVICES,VOL.45,NO.12,DECEMBER1998whichconsistsofholesinjectedfromtheextrinsicbaserailsintotheswitchcollector.Switchemitter/base(injectorbase/collector)deple-tioncharge Thiselementofchargeisdenedbytheactivearea.Switchcollector/basedepletioncharge Thiselementofchargeisdenedbytheswitchcollectorarea.Switchcollector/basesidewalldepletioncharge ThiselementofchargeaccountsfortheverticalsidewallsformedbytheP implantationsdowntotheSiGelayer.Thedepletionchargeintheemitter/basejunctionoftheinjectortransistorwasassumedtobenegligible.Thevoltagetermsincludedineachchargedifferenceexpressionaccountforthedifferenceinvoltagesatlogicª1ºandlogicª0.ºThefullexpressionsusedaregivenbelow: (A10) (A11) (A12) Theterms relatetohigh(1)andlow(0)logicconditionsand istheappropriatejunctionbuilt-involtage.SURFACEFEDI Holechargestoredinthe substrate Holechargestoredintheswitchemitter Electronsstoredintheswitchbase/injectorcollec- Holesstoredintheinjectorbase Holesstoredintheintrinsicswitchcollector Holesstoredintheswitchcollectorinjectedfromtheextrinsicbaserails Holesstoredintheinjectorbaseinjectedfromtheextrinsicbaserails Electronsstoredintheextrinsicbaserails Switchemitter/basedepletioncharge Switchcollector/basedepletioncharge Injectorcollector/basedepletioncharge Injectorcollector/basesidewalldepletioncharge (A25) WAINWRIGHTetal.:ANALYSISOFSi:GeHETEROJUNCTIONINTEGRATEDINJECTIONLOGIC2447Switchcollector/basesidewalldepletioncharge Thehigh-lowjunctionthatresultsifthesubstratedoping, ,isdifferentfromtheswitchemitterdoping, hasbeentreatedusingasimpliedversionoftheanalysispresentedbyDuttonandWhittier[16]whodeneablocking ,as: Theparameter [see(A2)]isthenwrittenas Physically,thisimpliesthatforalargerswitchemitterdoping thatthejunctionisªcollectingºandforasmallerswitchemitterdopingconcentration thatthejunctionisªre ecting.º[1]K.HartandA.Slob,ªIntegratedinjectionlogic:AnewapproachtoIEEEJ.Solid-StateCircuits,vol.SC-7,pp.346±351,1972.[2]H.H.BergerandS.K.Wiedmann,ªMerged-transistorlogic(MTL)ÐAlow-costbipolarlogicconcept,ºIEEEJ.Solid-StateCircuits,vol.SC-7,pp.340±346,1972.[3]H.H.BergerandK.Helwig,ªAninvestigationoftheintrinsicdelay(speedlimit)inMTL/IIEEEJ.Solid-StateCircuits,vol.SC-14,pp.327±337,1979.[4]D.D.Tang,T.H.Ning,R.D.Isaac,G.C.Feth,S.K.Wiedmann,andH.N.Yu,ªSubnanosecondselfalignedIL/MTLcircuits,ºIEEEJ.SolidStateCircuits,vol.SC-15,pp.444±449,1980.[5]B.MazhariandH.Morkoc,ªIntrinsicgatedelayofSi/SiGeinte-gratedinjectionlogiccircuits,ºSolid-StateElectron.,vol.38,no.1,pp.189±196,1995.[6]M.KarlsteenandM.Willander,ªImprovedswitchtimeofILatlowpowerconsumptionbyusingaSiGeheterojunctionbipolartransistor,ºSolid-StateElectron.,vol.38,no.7,pp.1401±1407,1995.[7]T.E.HendricksonandJ.S.T.Huang,ªAstoredchargemodelforestimatingILgatedelay,ºIEEEJ.Solid-StateCircuits,vol.SC-12,pp.171±176,1977.[8]V.Blatt,P.S.Walsh,andL.W.Kennedy,ªSubstratefedlogic,ºJ.Solid-StateCircuits,vol.SC-10,pp.336±342,1975.[9]F.M.Klaassen,ªDevicephysicsofIntegratedInjectionlogic,ºTrans.ElectronDevices,vol.ED-22,pp.145±152,1975.[10]S.P.Wainwright,S.Hall,andP.Ashburn,ªTheuseofMEDICIandSPICEinthedesignofsurfaceandsubstratefedintegratedinjectionlogic(IL)structuresincorporatingSiColloq.PhysicalModelingofSemiconductorDevices,pp.4/1±4/6,Apr.[11]S.E.Swirhun,Y.H.Kwark,andR.M.Swanson,ªMeasurementofelectronlifetime,electronmobilityandbandgapnarrowinginheavilydopedp-typesilicon,ºinIEDMTech.Dig.,1986,p.24.[12]J.delAlamo,S.Swirhun,andR.M.Swanson,ªSimultaneousmeasure-mentofholelifetime,holemobilityandbandgapnarrowinginheavilydopedn-typesilicon,ºinIEDMTech.Dig.,p.290,1985.[13]S.S.Iyer,G.L.Patton,J.M.C.Stork,B.S.Meyerson,andD.L.Harame,ªHeterojunctionbipolartransistorsusingSi-Gealloys,ºTrans.ElectronDevices,vol.36,pp.2043±2064,Oct.1989.[14]T.Nakamura,K.Nakazato,T.Miyazaki,M.Ogirima,T.Okabe,andM.Nagata,ª290psecILcircuitsusingve-foldself-alignment,ºinIEDMTech.Dig.,1982,p.684.[15]R.W.DuttonandR.J.Whittier,ªForwardcurrent-voltageandswitchingcharacteristicsofp(epitaxial)diodes,ºIEEETrans.Electron,vol.ED-16,pp.458±467,May1969. SimonP.Wainwright(S'91±M'95)wasborninNewcastle-Under-Lyme,U.K.,in1969.HereceivedtheB.Eng.(hons.)degreeinelectronicengineeringin1991andthePh.D.degreein1995fromtheUniversityofLiverpool,U.K..HisthesiswasbasedonSOItechnologyandincludedmaterialscharac-terization,devicephysicsandthedesignofverylowvoltage,lowpowercircuits.In1994hestartedstudyingtheSiGemateri-alssystemprimarilymodelingSiGeHBT'sandoptimisingtheircharacteristicsforinclusioninin-tegratedinjectionlogiccircuits.In1996,hemovedtoSpaintojointheUniversityoftheBasqueCountry(UPV/EHU)asaLecturer,whereheworkedonlowpowercircuitdesign.InMarch1998,hemovedintothesemiconductorindustrywithFagorElectronicaS.Coop.,Guipuzcoa,Spain,workingondiscretepowerdevices.Dr.WainwrightisamemberoftheInstituteofPhysicsandisaChartered StephenHall(M'93)receivedthePh.D.degreefromtheUniversityofLiverpool,U.K.,in1987,forworkonanewformofintegratedinjectionlogicintheGaAs/AlGaAsmaterialssystem.Hethenjoinedthelecturingstaff,UniversityofLiverpool,wherehebeganworkonSOImateri-alscharacterizationanddevicephysicsandsubse-quentlyobtainedfundingtoworkintheseareasontheSIMOXandoxidisedporousSisystems,withU.K.universityandindustrialcollaborators.OtherareasofactivityconcernedfabricationandelectricalassessmentofcobaltdisilicideSchottkydiodestogetherwithsilicon-germaniummaterialscharacterizationanddevicephysicsforbipolartransistorandMOSFETappplication.TheSiGeworkcurrentlyconcernshighperfor-manceanaloguebipolarandnewformsofintegratedinjectionlogicwithparticularinterestinmodelingandassociatedmaterialscharacterization.CurrentSOIworkisintheareaoflowvoltage/lowpowerintegratedcircuitswhichaimstoaddressthistopicfromdevicesandtechnologythroughlogiccircuitstylestoarchitecturesandalgorithms,incollaborationwithotherU.K.universitiesandindustry. PeterAshburn(M'89)wasborninRotherham,U.K.,in1950.HereceivedtheB.Sc.degreeinelectricalandelectronicengineeringin1971,andthePh.D.degreein1974,bothfromtheUniversityofLeeds,U.K.Hisdissertationtopicwasanexperi-mentalandtheoreticalstudyofradiationdamageinsiliconp-njunctions.In1974,hejoinedthetechnicalstaffofPhilipsResearchLaboratories,Redhill,U.K.,andworkedinitiallyonion-implantedintegratedcircuitbipolartransistors,andthenonelectronlithographyforsubmicronintegratedcircuits.In1978,hejoinedtheacademicstaffoftheDepartmentofElectronicsandComputerScience,UniversityofSouthampton,Southampton,U.K.,asaLecturer.Currently,heisaProfessoratthesameuniversity,wherehispresentareasofresearcharenpnandpnppolysiliconemitterbipolartransistors,Si/SiGeheterojunctionbipolartransistors,highspeedbipolarandBiCMOSprocesses,andsilicononinsulatortechnology.Hehasauthoredandcoauthoredover100papersinthetechnicalliteratureandhasauthoredabookonbipolartransistors. AndrewC.LambreceivedtheB.Eng.degreeincomputerandmicroelectronicsystemsfromtheUniversityofLiverpool,U.K.,inJuly1996,whereheiscurrentlyaResearchAssistantpursuingthePh.D.degree.HisresearchinterestsincludeSiGeheterojunctionbipolartransistors.