Superjunction-MOSFET

1.IntroductionInrecentyears,asconcernforprotectingtheglobalenvironmentalhasheightened,intheIT(in-formationtechnology)sector,greenIThasbeenat-tractingattentionasawaytoachievepowersavings.InordertoreducethepowerlossofITequipment,thepowerconvertersusedwiththeITequipmentmustbemademoreefficient.Consequentlythereisstrongdemandforlow-losspowerMOSFETs(Metal-Oxide-SemiconductorField-EffectTransistors).ThepowerMOSFETsusedinpowerconvertersoperateasswitchingdevicesandtheirassociateddissipationlossconsistsofconductionlosswhilethepowerMOSFETisON,andswitchinglosswhenthepowerMOSFETturnsonandturnsoff.Generally,theconductionlossisdominantinapplicationshavingalowswitch-ingfrequency,andtheswitchinglossisdominantinapplicationshavingahighswitchingfrequency.Theon-resistancenormalizedtoaunitarea(Ron·A)isusedasafigureofmeritfortheconductionloss,andthegate-to-drainchargenormalizedtotheon-resistance(Ron·QGD)isusedasafigureofmeritfortheswitchingloss.Thus,inordertoreducethedissipationlossofapowerMOSFET,minimizationofthesefiguresofmeritisstronglyrequested.However,thebreakdownvoltageandRon·Aareas-sociatedwithatheoreticallimitthatisdeterminedbythematerial(inthecaseofsilicon,thistheoreticallim-itisknownasthesiliconlimit),anditisnotthoughttobepossibletoobtainaRon·Avaluethatexceedsthistheoreticallimit.Thesuperjunction(SJ)structureisaninnovativebreakthroughthatovercomesthislimi-tation,andisattractingattentionduetoitsabilitytorealizedramaticallylowerRon·A(1)(2).ThispaperdescribesthefabricationmethodandcharacteristicsofFujiElectric’snewlydeveloped600V-classSJ-MOSFETthatrealizestheloweston-resistanceintheindustryandahighlevelofinductiveloadavalanchewithstandcapability.2.TechnicalTrendsofPowerMOSFETsFigure1showsthetrendsofRon·AandRon·QGDfor600V-classpowerMOSFETs.Asdescribedabove,atheoreticallimitexistsfortheRon·AofthepowerMOSFET,andthereforedevelopmenteffortsuntilnowhavefocusedonhowtomaketheRon·Aapproachthesiliconlimitascloselyaspossible.InordertoreducetheRon·A,thedriftresistance,whichformsthesiliconlimit,wasreducedandotherresistancecomponentswerealsoreduced.ThelatterentailsreducingthechannelresistanceandJFET(JunctionField-EffectTransistor)resistancebyimprovingthecelldensityFig.1PowerMOSFETRon·AandRon·QGDtrends0.00.20.40.60.81.01.220052010200019951990NormalizedRon∙A(a.u.)NormalizedRon∙QGD(a.u.)1985(Year)0.00.20.40.60.81.01.2Quasi-planejunctiontechnologySJ-MOSFETConventionalMOSFET65†AdvancedTechnologyLaboratory,FujiElectricHoldingsCo.,Ltd.‡SemiconductorsGroup,FujiElectricSystemsCo.,Ltd.YasuhikoOonishi†AkihikoOoi‡TakayukiShimatou‡SuperjunctionMOSFETABSTRACT600V-classsuperjunction(SJ)MOSFETs(package:TO-220)withamaximumon-resistanceof0.16Ωhavebeenfabricatedbyusingmulti-epitaxialgrowthtechnologywhichhasanexcellentcapabilityforcontrollingthedopingconcentration.ByoptimizingthedopingconcentrationintheSJstructure,thefabricatedSJ-MOSFETachievesanapproximate70%reductioninspecificon-resistancecomparedtothatofaconventionalMOSFET“SuperFAP-E3.”Thisistheindustry’shighestlevelofspecificon-resistance,anditsvalueexceedsthetheoreticallimitforconventionalMOSFETs.TheavalanchewithstandcapabilityofthefabricatedSJ-MOSFEThasbeenalsoimprovedovertheratedcurrentbyoptimizingthedopingprofileoftheSJstructureinthedepthdirectionandthethicknessandresistivityofthen-bufferlayer.ofthesurfaceMOSFETareaandoptimizingthecellstructure,andtheformerentailsreducingthedriftresistancebyoptimizingtheresistivityandthick-nessofthedriftlayerthatensuretheinductiveloadavalanchewithstandcapabilityandthebreakdownvoltage.Withastructurethatusesap-wellregiontoensuretheinductiveloadavalanchewithstandcapability,thereisalimittoextentwithwhichthedriftresistancecanbereduced,buttheapplicationofquasi-planejunctiontechnologyenablesthislimittobelowered(3).Withquasi-planejunctiontechnology,insteadofusingap-wellstructure,thedriftresistanceisreducedbynarrowingthedistancebetweenp-baseregionswithoutincreasingtheJFETresistance,andbyoptimizingthep-baseshape,aninductiveloadavalanchewithstandcapabilityequivalenttothatofap-wellstructureisguaranteed.Byapplyingquasi-planejunctiontechnology,theRon·AofaconventionalMOSFEThasbeenimprovedto110%ofthesiliconlimit,andtheRon·QGDhasalsobeenimprovedsignifi-cantlywiththereductioninRon·Aandthenarrowerdistancebetweenp-baseregions.FujiElectrichasappliedthisquasi-planejunctiontechnologytocom-mercializethe“SuperFAP-GSeries”whichisaneasy-to-usesuccessorofthe“SuperFAP-E3Series(4).”Recently,theSJ-MOSFET,whichbreaksthroughthesiliconlimit,hasbeenattractingattention.AsshowninFig.2,theSJ-MOSFETreplacesthep-typeandn-typeregionsinthedriftlayerofaconventionalMOSFETwithalternatingregionsofp-pillarsandn-pillars,andbecausetheimpurityconcentrationofthen-typeregionscanbeincreased,theRon·Acanbereduceddramatically.Moreover,sincetheRon·Acanbereduced,theactiveareacanbemadesmallerforthesameon-resistance,therebyenablingtheRon·QGDtobereducedaswell.3.SJ-MOSFETDevelopment3.1Multi-epitaxialgrowthtechnologyTheSJstructureenablesadramaticallylowervalueofRon·A,butbecauseitisachargecompensationstructure,ithasthedisadvant