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by  Alper, Cem

Modeling and optimization of Tunnel-FET architectures
exploiting carrier gas dimensionality
THÈSE NO 7587 (2017)
PRÉSENTÉE LE 24 MARS 2017
À LA FACULTÉ DES SCIENCES ET TECHNIQUES DE L'INGÉNIEUR
LABORATOIRE DES DISPOSITIFS NANOÉLECTRONIQUES
PROGRAMME DOCTORAL EN MICROSYSTÈMES ET MICROÉLECTRONIQUE
ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE
POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES
PAR
Cem ALPER
acceptée sur proposition du jury:
Dr G. Boero, président du jury
Prof. M. A. Ionescu, Prof. P. Palestri, directeurs de thèse
Dr U. E. Avci, rapporteur
Prof. M. Luisier, rapporteur
Dr J.-M. Sallese, rapporteur
Suisse
2017

Acknowledgements
ThisthesismarkstheendmyPhDjourneywhichtookabitmorethanfouryearsbutfelt
likeeternity. Indeed,workingonthesamesubjectforsuchalongtimesometimesgotabit
monotonousandfrustrating. Whatkeepsyougoingisthemomentswhenyouovercome
toughintellectualchallengesthatmakesyoufeelextremelyproudattheend.Overall,Iam
morethanhappytosaythatthelongworkpaidoffbigtime!
First, my gratitude and appreciation go to my thesis advisor Prof. Adrian M. Ionescu for
acceptingmeintohislabandallowingmetoworkonaveryfascinatingsubject.Hisextremely
smartobservationsandhisabilitytoseethe’bigpicture’helpedmesetmycoursebackon
trackwhenIgotlost. ManythankstoProf. PierpaoloPalestriofUniversityofUdinewho
actedasmyco-advisor.Iwasamazedtoseehiskeenattentiontodetailandhiswillingness
to get his hands dirty that saved my back so many times. It is truly an honor to witness
ourcollaboration,whichstartedasapseudo-randomencounter,developintoafull-fledged
student-tutorrelationship(thankyou,Skype).ThanksareduetoDr.Jean-MichelSallese,Dr.
GiovanniBoero,Prof. MathieuLuisier,Dr. UygarAvciforkindlyacceptingtobeinmyPhD
juryandtheirvaluableremarksonmythesis.
HavinganideallabenvironmentisessentialwhendoingaPhDwhichcanbedauntingat
times. I consider myself extremely lucky in this aspect, as I was surrounded by very nice
andsmartpeople.Ithankallmylabmembers(pastandpresent)includingLivio,Luca,Jose,
Nilay,Arnab,EmanueleandManeeshaforourcollaborationsandWolfgangandClaraforour
chitchatsandforlettingmeusethecouchwhenIwastooboredortiredtowork!Manythanks
toIsabelleandKarinfortheirpositiveattitudesandtheirhelpwiththeadministrativework.
WhenIfirstcametoLausannealmostsevenyearsagoformasterstudies,ImustadmitthatI
mainlyregardedLausanneasasteppingstonetotheUSforthePhD.Indeedmyplanseemed
toworkfineasIgotaverycompellingofferfromaworld-renownuniversityintheUSasaPhD.
However,bythetimeIwasfinishedwithmymasters,IwassohappyherethatIdecidedto
stay.Still,itwasthehardestdecisionImadeinmylifesofar(so,luckyme?).Icannotexpress
enoughhowhappyIamwiththeoutcome.IthankespeciallyBerker,KeremandEcefortheir
supportduringthetoughtimes.
Berkeralsodeservesaveryspecialmentionasheisaconstantsourceofcalmness,someoneto
talktowheneverIneeded;or,inshort,abrothertome.ManymanythankstoKeremforbeing
suchagoodfriendandalsoanexcellentlab/projectpartnerforagooddealofcoursessince
ourundergraduate.TheLausanneTurkishgangdeservesthehighestregards,theAkçokfamily
(Bas¸ak,˙IsmailandAlpin),Nariye,Meriç,Can,Bug˘ra,Gizem,Gizem,EceandEmre,youmade
iii
Acknowledgements
mytimeinLausanneworthwhile.Iwillneverforgetthenightsatour"MacLaren’s"(i.e.,Bar
Tabac).SpecialthankstoDilan,whohasbeenanexcellentandcaringfriendandanessential
partofourdomestictrio.
IcannotexpressmyfeelingsformybelovedHandanwithwords.Sheisanamazingwoman
whotrulyunderstandsme.Sheispatientenoughtoputupwithmyerraticbehaviorattimes.
Iamsoluckytohaveyoubymyside.IknowIwouldbelostwithoutyou.Thankyou.
Finally,IwanttothankmyparentsBirsen&Yusuf,mybrotherandsister-in-lawEmre&Belgin
andmyextendedfamilyfortheirconstantsupport. Ihavenotreceivedanythingbutlove,
warmthandsympathyfromthem.IameversogratefulformakingmewhoIamandproviding
aperfectlifeformeaswellassome(too)longnightsplayingvideogamesliketeenagers.
Lausanne,17February2017 CemAlper
iv
Abstract
Thesemiconductorindustry, governedbytheMoore’slaw, hasachievedthealmostunbe-
lievablefeatofexponentiallyincreasingperformancewhileloweringthecostsforyears.The
mainenablerforthisachievementhasbeenthescalingoftheCMOStransistorthatallowed
themanufacturerstopackmoreandmorefunctionalityintothesamechiparea.However,it
isnowwidelyagreedthatthehappydaysofscalingarewelloverandweareabouttoreach
thephysicallimitsoftheCMOSconcept. Onemajor,insurmountablelimitofCMOSisthe
so-calledthermionicemissionlimitwhichdictatesthattheswitchingslopeofthetransistor
cannotgobelow60mV/decatroomtemperature.Thismakesitimpossibletoscaledownthe
supplyvoltageforCMOStransistorwithoutdramaticallyincreasingthestaticpowerconsump-
tion.Toaddressthisissue,anoveltransistorconceptcalledTunnelFET(TFET)whichutilizes
thequantummechanicalband-to-bandtunneling(BTBT)hasbeenproposed.TFETspossess
thepotentialtoovercomethethermionicemissionlimitandthereforeallowforlowsupply
voltageoperation.
This thesis aims at investigating the performance of TFETs with alternative architectures
exploitingquantizedcarriergasesthroughquantummechanicalsimulations.Tothisend,1D
and2Dself-consistentSchrödinger-Poissonsolverswithclosedboundariesaredeveloped
alongwiththephonon-assistedanddirectBTBTmodelsimplementedasapost-processing
step. Moreover,weproposeanefficientmethodtoincorporatethequantizationalongthe
transversedirectionwhichenablesustosimulatedifferentdimensionalitycombinations.The
implementedmodelsarecalibratedagainstexperimentalandmorefundamentalquantum
mechanicalsimulationmethodssuchask·p andtight-bindingNEGFusingtunnelingdiode
structures.
Usingthesetools,wesimulateanadvancedTFETarchitecturecalledelectron-holebilayer
TFET(EHBTFET)whichexploitsBTBTbetween2Delectronandholegaseselectrostatically
inducedbytwoseparateoppositelybiasedgates.Thesubband-to-subbandtunnelingisfirst
analyzedwiththe1Dsimulatorwherethedeviceworkingprincipleisdemonstrated.Then,
non-idealitiesoftheEHBTFEToperationsuchasthelateraltunnelingandcornereffectsare
investigatedusingthe2Dsimulator.Theoriginofthelateralleakageandtechniquestoreduce
itareanalyzedindetail.
AparameterspaceanalysisoftheEHBTFETisperformedbysimulatingawiderangeofchannel
materials,channelthicknessandoxidethicknesses. Ourresultsindicatethepossibilityof
having2D-2Dand3D-3DtunnelingfortheEHBTFET,dependingontheparameterschosen.
AnoveldigitallogicschemeutilizingtheindependentbiasingpropertyoftheEHBTFETn-
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Acknowledgements
andp-gatesisproposedandverifiedthroughquantum-correctedTCADsimulations. The
performancebenchmarkingagainsta28nmFD-SOICMOStechnologyisperformedaswell.
TheresultsindicatethattheEHBTFETlogiccanoutperformtheCMOScounterpartinthelow
supplyvoltage(subthreshold)regime,whereitcanoffersignificantlyhigherdrivecurrentdue
toitssteepswitchingslope.
We also compare the different dimensionality cases and highlight important differences
betweenthefaceandedgetunnelingdevicesintermsoftheirdependenceonthedevice
parameters(channelmaterial,channelthicknessandEOT).
Keywords:TunnelFET,quantumsimulation,steepslopeswitch,circuitbenchmarking,band-
to-bandtunneling,analyticalmodeling,numericalsimulation,finiteelementmethod,carrier
gasdimensionality,densityofstates(DOS),2D-2Dtunneling,counterdoping,hetero-gate,
electron-holebilayer
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Sommario
BasandosisullaleggediMoore,l’industriadeisemiconduttorièstataingradodimetterein
attoun’impresaquasiincredibile:aumentareesponenzialmenteleprestazionideidispositivi,
congiuntamenteallariduzionedeicosti.Questoèstatopossibilegraziealprocessodiminia-
turizzazionedeltransistoreCMOS,chehapermessodiaumentaresempredipiùladensità
didispositiviperunitàd’area.Tuttavia,èormaicomunementeaccettatoilfattochequesto
processostiagiungendoauntermine,essendoalpuntodiraggiungereilimitifisicidellatec-
nologiaCMOS.Adesempio,unlimiteinsormontabilerelativoaquestatecnologiaèdovutoalla
cosiddettaemissionetermoionica,chefasìchelapendenzadisottosoglianonpossaessere
inferiorea60mV/decatemperaturaambiente.Perquestomotivo,risultaimpossibileridurre
latensionedialimentazioneperitransistoriCMOSsenzaaumentareesponenzialmenteil
consumodipotenzastatico.Perrisolverequestoproblema,èstatopropostountransistore
alternativochiamatoTunnelFET(TFET),basatosulmeccanismoditunnelingquantistico
banda-banda.ITFEToffronolapossibilitàdisuperareillimitedell’emissionetermoionicae
cosìpermetterediridurrelatensionedialimentazione.
L’obiettivodiquestatesiconsistenellostudiodelleprestazionidiTFETconarchitetturealter-
native,basatesugasdiportatoridicaricaquantizzati.Lostudioèstatoeffettuatoattraverso
simulazioni numeriche, includendo effetti di meccanica quantistica. A tal fine, sono stati
sviluppatiprogrammiperrisolvereinmanieraautoconsistenteproblemiSchrödinger-Poisson
1De2D,congiuntamenteall’usodimodelliditunnelingquantisticobanda-bandadirettoo
assistitodafononi,applicatiinunafasesuccessiva.Inoltre,inquestatesiproponiamoun
metodoperconsiderarelaquantizzazionenelladirezionetrasversa,checipermettedisimu-
lareefficientementediversecombinazionididimensionalità.Imodelliimplementatisono
staticalibratiavendocometerminediparagonedatisperimentaliemetodidisimulazione
alternativibasatisusimulazionidimeccanicaquantisticafondamentale,comeilk·p eil
tight-bindingNEGFbasatosustrutturedidiodoaeffettotunneling.
Tramitequestistrumenti,abbiamosimulatoun’architetturaTFETavanzatadenominatoTFET
adoppiostratoelettrone-lacuna(electron-holebilayerTFEToEHBTFET),chesibasasul
tunnelingbanda-bandatragasbidimensionalidielettronielacune,indottiinmodoelettro-
staticodadueterminalidigatedispostioppositamente.Primadituttoabbiamoanalizzato
iltunnelingdasottobandaasottobandaconunsimulatoremonodimensionale,alfinedi
dimostrareilprincipiodifunzionamentodeldispositivo.Successivamenteilsimulatore2Dè
statoimpiegatoperstudiaregliaspettinon-idealidell’EHBTFET,comeiltunnelinglaterale
e gli effetti agli angoli. Lecause del tunnelinglaterale eletecnicheperridurlo sono state
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Acknowledgements
analizzateindettaglio.
Abbiamoeffettuatounostudiodeiparametridiprogettodell’EHBTFETsimulandounavasta
gammadimaterialiperilcanale,edispessoriperilcanaleeperildielettrico.Inostririsultati
dimostranocheasecondadeiparametril’EHBTFETpuòesibiretunnelingsia2D-2Dche
3D-3D.InoltreabbiamopropostoeverificatoattraversosimulazioniTCADconcorrezioni
quantisticheunoschemalogicodigitaleinnovativobasatosulleproprietàdipolarizzazione
indipendentedeigateditiponep.dell’EHBTFET.Inoltreabbiamoconfrontatoleprestazioni
dell’EHBTFET rispetto a quelle offerte dalla tecnologia FD-SOI CMOS a 28 nm. I risultati
mostranocheunalogicabasatasull’EHBTFETpuòforniremiglioriprestazionirispettoalla
logicaCMOSinregimedibassatensionedialimentazione,dovelacorrentepuòesserepiù
altaacausadellamaggiorependenzasottosoglia.Abbiamoancheconfrontatodiversicasi
didimensionalitàesottolineatoimportantidifferenzetradispositivibasatisutunnelingdi
superficieodispigoloinquantoadipendenzasuparametridiprogetto(materialeespessore
delcanale,spessoreeffettivodeldielettrico).
Parolechiave:tunnelFET,simulazionequantistica,interruttoreadaltapendenzasottosoglia,
analisicomparativadicircuiti,tunnnelingbanda-banda,modellizzazioneanalitica,simula-
zionenumerica,metodoadelementifiniti,dimensionalitàdigasdiportatoridicarica,densità
distati(DOS),tunneling2D-2D,controdopaggio,eterogate,doppiostratoelettrone-lacuna.
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List of Publications
Partsofthisthesishavebeenpublishedinthepaperslistedbelow:
1. C.Alper,J.L.Padilla,P.Palestri,A.M.Ionescu."Anovelreconfigurablesub-0.25Vdigital
logicfamilyusingtheelectron-holebilayerTFET,"IEEETransactionsonElectronDevices,
Inpreparation.
2. D.Esseni,P.Palestri,M.Pala,C.Alper1,T.Rollo."Areviewofselectedtopicsinphysics
basedmodellingofsmallslopetransistorsbasedonband-to-band-tunnelling,"Semi-
conductorScienceandTechnology,Inpreparation.
3. C.Alper,J.L.Padilla,P.Palestri,A.M.Ionescu. "Impactofdevicegeometryofthefin
Electron-HoleBilayerTunnelFET,"ESSDERC2016-46thEuropeanSolidStateDevice
ResearchConference,Sept.12-15,2016.
4. C.Alper,P.Palestri,J.L.Padilla,A.M.Ionescu."TheElectron-HoleBilayerTFET:Dimen-
sionalityEffectsandOptimization,"IEEETransactionsonElectronDevices,63(6),pp.
2603-2609,2016.
5. C.Alper,P.Palestri,J.L.Padilla,A.M.Ionescu."Underlapcounterdopingasanefficient
meanstosuppresslateralleakageintheelectron-holebilayertunnelFET,"Semiconduc-
torScienceandTechnology,31(4),pp.045001,2016.
6. C.Alper,M.Visciarelli,P.Palestri,J.L.Padilla,A.Gnudi,E.Gnani,A.M.Ionescu."Mod-
elingtheimaginarybranchinIII-Vtunnelingdevices:Effectivemassvsk·p,"SISPAD
2015,Sept.9-11,2015.
7. C.Alper,P.Palestri,J.L.Padilla,A.Gnudi,R.Grassi,E.Gnani,M.LuisierandA.M.Ionescu.
"Efficientquantummechanicalsimulationofband-to-bandtunneling,"EUROSOI-ULIS
2015,Jan.26-28,2015.
8. A. M. Ionescu, C. Alper, J. L. Padilla, L. Lattanzio, P. Palestri. "Electron-hole bilayer
deepsubthermalelectronicswitch: Physics,promiseandchallenges,"2014SOI-3D-
SubthresholdMicroelectronicsTechnologyUnifiedConference(S3S),Oct.6-9,2014.
9. C. Alper, P. Palestri, L. Lattanzio, J. L. Padilla and A. M. Ionescu. "Two dimensional
quantummechanicalsimulationoflowdimensionaltunnelingdevices,"SolidState
Electronics,113,pp.167-172,2015.
1FinalauthororderingTBD.
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ListofPublications
10. C.Alper,L.DeMichielis,L.Lattanzio,P.Palestri,L.SelmiandA.M.Ionescu."Quantum
mechanicalsimulationofthegermaniumelectron-holebilayerTFET,"IEEETransactions
onElectronDevices,60(9),pp.2754-2760,2013.
Acompletelistofpublicationscanbefoundattheendofthethesis.
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