INVESTIGATION OF DEVICE ENGINEERED INNOVATIVE TUNNEL FETs FOR IMPROVED PERFORMANCES

Abstract

Technologicaldevelopmentofthesemiconductorindustryisprimarilydependedonthe miniaturization ofexistingCMOStechnologywithoutaffectingitsproperfunctionalityor in otherwords,morefunctionalityperunitareaofsilicon.However,thepowerconstraints and ever-increasingleakagecurrentinscaledtransistorshaspromptedtheresearchonin- ventivesteepslopedevices.Amongtheemergingdevices,TunnelFieldEffectTransistor (TFET) isoneofthemostpromisingandpotentialcandidateswhichcouldbeapossible successor toMOSFET.DuetothevirtueoflowOFFcurrentandsteepsubthresholdslope, TunnelFETscanbeusedforlowpowerapplications.Inspiteofhavingsuchadvantages, TunnelFETssufferfromtheproblemofambipolarconduction,lowONcurrentandhigher Miller capacitance.ManyliteratureshavereportedstructuralandmaterialengineeredTunnel FETs; however,extensiveresearchisstillrequiredforbetterdeviceperformance.Inorderto improvethedevicecharacteristics,wehaveproposedfourdistinctnovelTunnelFETstruc- tures inthisthesiswithproperphysicaljustifications,optimization,andcomparisonwiththe existingavailableliterature. WeproposedalinetunnelingbasedTwoSourceRegion(TSR)TunnelFETinwhich the effectivetunnelingoccursatthejunctionofthechannelandboththesourceregions. The proposeddeviceisasilicon-on-insulator(SOI)basedstructurethatcomprisesadrain pad overwhichboththesourceregionsandchannelareformed.Awell-calibratedsimula- tion setuphasbeenusedtoobtaintheresultsoftheproposedstructure.Wealsoexplained the possibleprocessflowtofabricatetheproposedstructure.Theacquiredresultsarecom- pared withtheearlierreportedL-shapedTunnelFETandwehavefound 63% reduction in turn-onvoltagewith 100 timesimprovementin ION=IOFF. Thedevicedimensionsand other parametersareoptimizedwithappropriatereasoningtoimprovetheperformanceof the proposeddevice. Literature surveyofTunnelFETssuggeststhatthepointtunnelingisnotadominant mechanism asitoccursonlyinthelocalizedareanearthesurface.Totakethisintocon- sideration, wehavechosenagate-all-aroundstructuretoimprovetheONcurrentincaseof point tunneling.WeproposedaGateAllAroundSchottkyJunctionTunnelFETwithHeav- ily DopedPocket(GAASJ-HDP)inwhichtheSchottkyJunctionhasbeenformedatthe source-channel interface.TheexistenceofSchottkyJunctionsupportstogetasteeptunnel- ing widthandinturn,increasethetunnelingcurrent.Theresultsoftheproposedstructure are comparedwithaconventionalgate-all-around(GAA)TunnelFETandwehavefound 15x improvementinONcurrent.Theimpactofheavilydopedpocket(HDP),stacked gate-oxideandchannellengthvariationintheproposedstructurehavealsobeendiscussed in detail.Furthermore,theconceptofunder-lapandthedualmetalgatehasbeenincluded in theproposedstructuretoinvestigatetheperformanceintermsofambipolarcurrent.In continuation, wedemonstratedaworkfunctionengineeredGateAllAroundTripleMetal (GAATM)TunnelFETwhichcomprisesofagateelectrodewiththreemetalsofdifferent i workfunctionconnectedincascade.Thechoiceofworkfunctionofthreedifferentmetals enhances theelectricfieldatthesource-channeljunctionwhichincreasestheONcurrent. Moreover,thedifferenceinworkfunctioncausesaformationofapotentialbarrierinthe channel whichobstructsthereversetunnelingandinturnreducestheambipolarcurrent.An analytical modeloftheproposeddevicehasbeenpresentedbyusingPoisson’sEquationand Kane’sModelandfoundtohaveagoodagreementwiththesimulationdata.Toemploythe device-circuitco-design,weproposedandinvestigatedanovelTwofoldTunnelFET(TF- TFET). TheproposedTF-TFETisanadmixtureofbothn-andp-typeTunnelFETsandthus a singledeviceactsasaninverter.Thisinturnreducesthetransistorcountindesigningof anycircuit.Thedrainregionofn-andp-typeTunnelFETsareshortedinternallywhichpro- vides anadditionalintrinsiccapacitancethatcomesinserieswiththeparasiticcapacitance of individualTunnelFET.Asaresult,wefound 4x reductionintheoverallMillercapac- itance ascomparedtotheMillercapacitanceofaconventionalTunnelFETbasedinverter. In addition,wehaveexploredthedynamicandstaticbehavioroftheTF-TFETinverterand also designedanXORgateand2:1MUXbyusingtheproposedTF-TFET.Inthisway,we haveacquiredthepossiblesolutionsbydemonstratingfourdistinctstructureswhichprovides better performancesofTunnelFETsintermsofhighONcurrent,lowambipolarcurrentand reduced Millercapacitance.Theoutlineoftheworksuggeststhattheproposeddevicescan be usefultothedeviceandcircuitengineerstoachieveimprovedperformancesofTunnel FET devices.