Reliability Characterization of GaAs FET TestStructures.docx
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Reliability Characterization of GaAs FET TestStructures.docx
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ReliabilityCharacterizationofGaAsFETTestStructures
RELIABILITYCHARACTERIZATIONOF
GaAsFETTESTSTRUCTURESFORAPPLICATIONSINRF/MICROWAVEMODULES
AshokK.SharmaMuzarA.Jah
NASAGoddardSpaceFlightCenter
ComponentTechnologiesandRadiationEffectsBranch
Code562
Greenbelt,MD20771
TABLEOFCONTENTS
I.INTRODUCTION3
II.GaAsFET-BASEDTECHNOLOGYOVERVIEW4
III.CHFETTECHNOLOGY5
IV.MAJORRELIABILITYISSUESFORGaAsFETSTRUCTURES7
OhmicContactDegradation8
GateContactandChannelDegradation8
Trap-RelatedEffects8
HydrogenPoisoning8
V.CHFETRELIABILITYSTUDIES9
VI.CHFETTHERMALCYCLINGTESTPROCEDURES10
VII.CHFETTHERMALCYCLINGRESULTS13
VIII.SUMMARY17
IXRECOMMENDATIONS…………………………………………………………17
APPENDIX20
I.
INTRODUCTION
Microwavetechnology,withitsvastbandwidthcapability,isthecornerstoneofmoderncommunicationssystems.Microwavesystemsapplicationsindomesticandinternationalsatellitecommunicationsincludepersonalcommunicationssystems,weatherforecasting,television,andnationaldefenseapplicationssuchasreconnaissance,aircraftandmissiledetection,andguidanceandcontrolsystems.TheprincipalreasonforfabricatingmicrowavedevicesoutofGalliumArsenide(GaAs)isgreaterspeedandperformance(duetoitshigherelectronmobility)ascomparedtocommonlyusedtechnologiesbasedonSilicon(Si).
GaAshasbecomethematerialofchoiceformanufacturersandcustomersinthefabricationofMonolithicMicrowaveIntegratedCircuit(MMIC)devicesforseveralimportantreasons,includingthefollowing:
1.Greaterspeedinperformance,realizedasmaximumfrequencyofoperationorhigherlogicswitchingspeeds.
2.Widerfrequency-bandwidthperformancefromthereductionofparasiticsindiscretedevicepackaging.
3.Highresistivitysemi-insulatingpropertythatreducescross-talkbetweendevices.
4.Costreductionformedium-tolarge-scaleproductionvolumes.
GaAs-basedmicrowavedevices,suchaspowerandlownoisemetalsemiconductorfieldeffecttransistors(MESFETs),heterostructureFETs(HFETs),andpseudomorphichighelectronmobilitytransistors(PHEMTs)havebeenusedandcontinuetooffersignificantperformanceenhancementsforapplicationsinsatellite-andground-basedcommunicationsystemsintheKu,K,andKabands(12to40GHz).Thesedeviceshavealsoshownpromiseforuseinmillimeterwaveband(x>40GHz),astechnologyimprovementsallowdevicefeatures,suchasthegatelengths,toshrink.Themaindevicefeaturelimitinghighfrequencyperformanceisthelengthandwidthofthegate.Thegatelengthsetsthehighfrequencylimit,whilethegatewidthdefinesthetransconductance(gm)andthedrain-sourcesaturationcurrent(IDSS).Devicemanufacturersstrivetodecreasethelengthofthegateforoperationsintothemillimeterwavefrequencyband.GaAsdevicesarerequiredinbothreceiveandtransmitfunctionsofspacecommunicationssystems;therefore,itisextremelyvitalforthespaceindustrytounderstandthephysicsandoperationsofthesedevices.
Althoughallcustomersbenefitfromtheseadvantages,themajorityofdevicesandcircuitsmanufacturedareproducedfromSiforcommercialapplicationsandaretestedforreliabilityaccordingly.ThedegradationandfailuremechanismsofSi-baseddevicesarewellunderstood,whileGaAsreliabilityissuesarefarfrommaturityandarestillunderinvestigation.Spaceapplications,whichcanrequireconfidenceindeviceperformanceformissiondurationsvaryingfromafewweekstoseveralyears,inadditiontoharshenvironments,requireextensivereliabilitytestingtoensurecost-effectiveprojectplanningandmissionsuccess.
Manyofthetestscurrentlyconductedbymanufacturersdonotguaranteethereliabilityoftheirdevicesaccordingtothestringentrequirementsnecessaryformilitaryandspaceapplications.AlthoughRFacceleratedlifetestingoftheRFdeviceswouldbeoptimal,itisquitecostly.AmuchsimplerDCtestingcanbeusedasamodestestimatetopredicttheRFdeviceperformance.Forexample,IDSScanbeusedasanindicatorofsaturatedpowerperformance,andgmcanbeutilizedtopredictgainandnoisefiguredegradationinsmallsignalandlownoisedevices.OnceadevicepassesDCtesting,itcanbefurthercharacterizedtostudytheeffectsoftemperaturecycling,acceleratedlifetestingwithRFparametricmeasurements.ThegoalofthistaskwastoperformthermalcyclingcharacterizationofGaAscomplementaryheterostructureFETs(CHFETs)procuredfromHoneywellSolidStateElectronicsCenter(SSEC)andtodeterminetheirperformancereliabilityinapplicationsutilizingRF/microwavetransmit/receive(T/R)modules.
II.GaAsFET-BASEDTECHNOLOGYOVERVIEW
Asstatedearlier,GaAs-basedFETsandhighelectronmobilitytransistors(HEMTs)currentlymakeupthevastmajorityofdevicesutilizedinhighfrequencyandhighpowerapplications.ThemainfeatureoftheHEMTistheformationofthe2-Delectrongasformedbythespatialseparationbetweenthechargecarriersintheactivechannelandtheirparentimpuritiesconfinedinthedonorlayer.Mostrecently,pseudomorphichighelectronmobilitytransistor(PHEMT)technology,attainingsuperiormobilitytostandardFETsandHEMTsthroughanAlGaAs/InGaAs/GaAsstructure,hasbeenintegratedintoRF/microwavemodules.MostdevelopersofGaAs-baseddevicesarenowfocusingonthefurtherdevelopmentandimprovementofPHEMTdevices.
ManufacturerscurrentlyproducingGaAs-baseddevicesformicrowaveapplicationsincludeAgilent,Raytheon,TriQuint,andWINSemiconductor.Table1providesanoverviewoftheirtechnologyandprocesscapabilities.
Table1.GaAsdiscretedevicemanufacturer’stechnologyandprocesscapabilitiesoverview.
*Informationnotavailable.
LowNoise
Amplifiers
PowerAmplifiers
Manufacturer
Gate
Length
(μm)
Gate
Width
(mm)
FrequencyRange
NoiseFigure
(dB)
Gain
(dB)
TriQuint
0.5–0.25
0.3–24
DC-22GHz
1.2
1.5–4
Raytheon
0.25
*
4–40GHz
1.4–2.7
N/A
Agilent
0.2
0.4–1.6
450MHz–10GHz
0.4–2.2
N/A
WIN
Semiconductor
0.5–0.15
0.3–1.2
15–100GHz
0.4–1.0
18
III.CHFETTECHNOLOGY
CHFETisanadvancedGaAs-basedICtechnologythatoffersperformanceseveraltimesbetterthanSi-basedCMOScircuits.ItsuniquenessliesinthefactthatitisacomplementaryprocessbasedonGaAsasopposedtoSi.Itoffersuptofourtimeshigherspeedanddrawsone-sixthofthepowerrequiredbySi-basedCMOSdevices.Inaddition,CHFETshaveinherentradiationhardnessandaresuitableinhighperformancephotodetectorsforintegratedoptoelectronicapplications.SuperiorperformanceofGaAsCHFETtechnologyalsoincludeslowstaticpowerdissipation.Inanidealcase,CHFETcircuitsdissipatezerostaticpoweranddissipatepoweronlyduringtheswitchingprocess.However,theattainmentofthelowstaticpowerrequiresboththen-andp-channeldevicestobeenhancement-modedevices.Theirthresholdvoltagesaretypicallysettobe20%ofthesupplyvoltage,sothatneitherdeviceisturnedonforquiescenthighorlowinputvoltages.Gateleakagecurrentmustalsobelimited,soasnottoaffectthelogicswingofthecircuitorcontributetothestaticpowerdissipation.
TheadvantageofCHFETcircuitscomesfromthehigherattainablecarriermobilitiesinIII-Vcompounds.InthecaseofGaAs,theelectronmobilityismuchhigherthanthatoftheSi,whiletheholemobilityismuchlower.Asaresult,thep-channeldeviceslimitthespeedofGaAs-basedcomplementarycircuitry.Therefore,n-channelcircuits,suchastheMESFETanddirect-coupledFETlogic(DCFL),havebeengenerallypreferred.Enhancedholetransportinp-channelAlGaAs/GaAsHEMTshasincreasedtheattentiononvariousCHFETapproaches.CHFETarchitecturesbaseduponAlGaAs/GaAsHEMTs,metalinsulatorsemiconductorFETs(MISFETs),semiconductorinsulatorsemiconductorFETs(SISFETs),andquantumwell(QW)-MISFETsarecurrentlybeingexplored.
TheCHFETisanenhancementmodedevicewhoseoperationisanalogoustothatofaSiliconCMOSFET.Applyingasuitablegatevoltageinducesachargetoaccumulateinthechannelbydrawingcarriersfromthesource:
electronsinthecaseofn-channeldevicesandholesinp-channeldevices.Thedielectric(forexample,undopedAlGaAsinsulator/barrierlayer)playsaroleanalogoustotheoxideinaMOStransistor.Itprovidesapotentialbarrierthatconfinesthechannelcharge,preventingconductionbetweenthechannelandthegate.Itisexpected,however,thatsomeleakagecurrentthroughthegateexists,causedbyfieldemission(quantummechanicaltunnelingofthecarriersthroughthepotentialbarrier)andthermionicemissionofthecarriersoverthebarrier.Themagnitudeofthiscurrentislikelytodependstronglyonthegatevoltageandthedevicetemperature.
HoneywellGaAsCHFETtechnologyincorporatesanAl0.75GaAs/In0.25GaAs/GaAsheterostructureandbandgapengineeringtechniquestomaximizegateisolation.TheInGaAschannelisusedbecauseofthehigherelectronvelocity[1].Figure1representsthetypicaln-andp-channelstructureusedintheHoneywellGaAsCHFETtechnology.
250ÅAl0.75GaAs
150ÅIn0.25GaAschannel
0.5μmGaAsbuffer
Figure1.BasicHoneywellCHFETstructure.
TheAl0.75GaAslayer(withanenergybandgapofapproximately2eVatroomtemperature)actssimilarlytothegateoxideinSi-CMOS,reducingthegateleakagecurrentfromthatofacomparablebulkGaAsMESFET.TheIn0.25GaAslayerformsthechannelofthedeviceandprovidesa30%enhancementinlatticemobilitiesascomparedtobulkGaAsforbothelectronsandholes.Asilicondelta-dopedlayerislocatedbelowtheInGaAschanneltoprov
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