電子器件-7-MOS-2課件

Chapter6.5:TheMOSFieldEffectTransistorPresentedby:LeiWang6.5.5ControlofthresholdvoltageChoiceofgateelectrode:Al,WSix,p+-Si,etc.d:thicknessi:SiO2,SiNOx,Ta2O5,ZrO2,PZT26.5.5ControlofthresholdvoltageControlofCi:(1)dthicknessofgateoxideandfieldoxide(2)highimaterial:Ta2O5,ZrO2andferroelectricsGateoxideandfieldoxide36.5.5ControlofthresholdvoltageProblem:Considerann+polysilicon-SiO2–Sip-channeldevicewithNd=1016cm-3andQi=5×1010qC/cm2.CalculateVTforagateoxidethicknessof0.01μmandrepeatforafieldoxidethicknessof0.5μm.Фms=-0.25V.Solutions:VTforagateoxidethicknessof0.01μmis-1.1V,andforafieldoxidethicknessof0.5μmis-9.1V.56.5.5Controlofthresholdvoltage66.5.5ControlofthresholdvoltageIonimplantation(離子注入)mainparameters:implantationenergy(注入能量)keVimplantationdose(注入劑量)ions/cm2Neff=Nd-NaVTbecomeslessnegative7Example6-3Forap-channeltransistorwithagateoxidethicknessof10nm,calculatetheboroniondoseFB(B+ions/cm2)requiredtoreduceVTfrom-1.1Vto-0.5V.AssumethattheimplantedacceptorsformasheetofnegativechargejustbelowtheSisurface.If,insteadofashallowBimplant,itwasamuchbroaderdistribution,howwouldtheVTcalculationchange?Assumingaboronionbeamcurrentof10-5A,andsupposingthattheareascannedbytheionbeamis650cm2,howlongdoesthisimplanttake?9Somequestions1.MOSFET器件的平帶電壓是如何產(chǎn)生的?2.非理想情況下,MOSFET的閾值電壓由哪幾項組成?它們與什么參數(shù)有關(guān)?3.采用什么方法和手段可以控制閾值電壓?4.MOSFET中描述其特性的一個參數(shù)為跨導(dǎo)gm,它描述了器件的什么特性?10KeyWordsStandby 待機reciprocal 倒數(shù)scalingdown 等比例縮小powerdissipation 功耗packingdensity 集成密度operationspeed 工作速度channelhotelectroneffect 溝道熱電子效應(yīng)substratehotelectroneffect 襯底熱電子效應(yīng)MilleroverlapCapacitance 米勒覆蓋電容116.5.6SubstratebiaseffectsIntheabove,thesourceisassumedtobeconnectedtothesubstrate.136.5.6SubstratebiaseffectsNow,weapplyareversevoltagebetweenthesourceandsubstrate.HowaboutVT?146.5.6SubstratebiaseffectsSource=0VBF<0.6V156.5.6SubstratebiaseffectsP309

Withareversebiasbetweenthesubstrateandthesource,thedepletionregioniswidenedandthethresholdgatevoltagerequiredtoachieveinversionmustbeincreasedtoaccommodatethelargerQd.17QuestionHowaboutapplyingaforwardbiasbetweenthesourceandthesubstrate?18Conclusions:MOSFET正常工作時，必須保證漏p-n結(jié)和源p-n結(jié)零偏或反偏。有襯底偏置電壓時，閾值電壓絕對值增加。??196.5.7Sub-thresholdcharacteristicsVGfromVTtolesspositive,VFB

stronginversion

weekinversiondepletionflatband(electrondensityonthesurfaceequalstothedensityinbulk216.5.7Sub-thresholdcharacteristicsVGVT,weakinversionoccursThiscurrentisduetoweakinversioninthechannel.Itleadstoanelectron

diffusioncurrentfromsourcetodrain.226.5.7Sub-thresholdcharacteristicslnIDVG

ID

的求解參見<<微電子技術(shù)基礎(chǔ)>>曹培棟編著電子工業(yè)出版社249-252頁236.5.7Sub-thresholdcharacteristicsS,ameasureoftheefficacyofthegatepotentialinmodulatingID.ThesmallerthevalueofS,thebetterthetransistorisasaswitch.Sisimprovedbyreducingthegateoxidethickness.Sishigherforheavychanneldopingorifthesilicon-oxideinterfacehasmanyfastinterfacestates.256.5.7Sub-thresholdcharacteristicsForverysmallgatevoltage,sub-thresholdcurrentisreducedtotheleakagecurrentofthesource/drainjunctions.Thisdeterminestheoff-stateleakagecurrent,andthereforethestandbypowerdissipationinmanycomplementaryMOScircuits.VTistypicallydesignedtobe0.7V.266.5.8EquivalentcircuitfortheMOSFET276.5.8EquivalentcircuitfortheMOSFET296.5.8EquivalentcircuitfortheMOSFETMillercapacitancecanbereducedbyusingself-alignedgate.Becauseofthespreadofthesource/drainjunctionsunderthegate,thechannellengthisreducedbyΔL,theeffectiv