chapter-6-TFT-LCD

TFT-LCD1Chapter6contents2WhyTFTusedTFT-LCDoperationprincipleTFTTechnology1WhyTFTused3•DirectAddressing•PassiveAddressing•ActiveAddressingAddressing:DriveSchemesThresholdvs.Non-ThresholdDirectDriveAddressingThresholdlessnatureofmaterialisirrelevantEverypixelisindependentlyaddressedForlowerresolutionsonly50pixelsinch••MultiplexedAddressingCanaddressNMpixelsusingonlyN+Melectricalconnections(mediumresolution~400rows)StrictlimitationonthresholdvoltageandT-VsteepnesscurveVoltagesappliedtoonepixelcannotbearbitrarilychangedwithoutaffectingtheappliedvoltageoftheothercellsResponsetime,governedbyviscoelasticproperties,mustbethanperiodofdrivingwaveformIneachmultiplexingcycle,eachrowisselectedonduring1/NofthecycletimeTOFFTHVVONTHVV+ΔVTH:thresholdvoltage(turnonbegins)D:steepnessparameterTHΔPVTHMAXΔ1VNAltandPleshkoIEEETrans.ElectronicDevicesED-21,146-155(1974)Multiplexing:PracticalApplicationsPassiveMultiplexing:AmplitudeModulation9Nlimitation11ActiveMatrixDisplaysMultiplexingislimitedandnotadequateforhighresolutions(slowresponse,poorviewingangle,poorgrayscale)Anon-linearelementisbuildintothesubstrateateachpixel,usuallyathin-film-transistorBeingisolatedfromotherpixelsbyTFT’s,thevoltageremainsconstantwhiletheotherpixelsarebeingaddressed2TFT-LCDoperationprinciple13HowdoesaThinFilmTransistor(TFT)operate?ThinFilmTransistorBasedona-Si(orPoly-)insteadofcrystallineSiDepositedinthinlayersonaninsulatingsubstrate(glass)Applicationsswitch(-processors,AMLCD)Amplifier(drivers,analogcircuits)0510152025303540051015Vds[V]draincurrent[uA]2)(2thGifeDVVLWCIIdealTFTcharacteristicsfordifferentgatevoltagesThecurrentsaturatesatVDS≥VG–VthHighVGLowVGsaturationmode(1)ICWLVVVDfeiGthDS()VDSVG-Vth(linearregime)(2)ICWLVVVVDfeiGthDSDS()22VDSVG-Vth(sub-saturation)(3)ICWLVVDfeiGth22()VDSVG-Vth(saturation)SummaryTFToperationequations(validforcrystallinesilicon)See:S.M.Sze,“Semiconductordevices,physicsandtechnology”,Wiley&Sons,1985.ActiveMatrixAddressing2020/5/2017AMLCDEquivalentCircuit2020/5/2018ActiveMatrixCircuitScanLineSourceDrainLiquidCrystalBasicprincipleofTFTdrivinggatesource(-),databusdrain(+),currentelectronspixelThinFilmTransistor(TFT)TFTcross-sectionBasicprincipleofTFTdrivingPIXELCONFIGURATIONScanlinelight-shieldTFTcross-sectionLINEBYLINESELECTIONACTIVEMATRIXDRIVINGOFLCDSOperationPrincipleofTFT-LCDpixel(1)2020/5/2024ProcessofdatainputandholdisoneofchargeanddischargeforTFTswitchon/offChargeprocess:tochargeLCcapacitanceandstoragecapacitanceduringT1(ontime)Dischargeprocess:duringT2(offtime).ControlparameterstoholdmaximumchargestoavoidcrosstalkOperationPrincipleofTFT-LCDpixel(2)2020/5/2025Chargeprocess2020/5/2026ontIDLCeVV1SLCononCCRRon-resistanceofonstateCs－storagecapacitanceDataof99mustbeinputinTFTduringinT1IDCLVV%99)(6.46.41SLCononCCRTDischargeprocess2020/5/2027offtIDLCeVV＝)(SLCoffoffCCRIfcontroldatasignallosslessthan5IDLCVV05.05.19)(5.192SLCoffoffCCRT＝RoffresistanceonoffstateRoff/RonratioofTFT2020/5/2028ForgivenTVsystem,T1，T2knownparametersRoff/Ron105NoscanlimitationHighresolutionandfastresponsespeedLCCapacitanceandStorageCapacitance2020/5/2029BasicstructureforCs2020/5/2030CsITOforDisplayParasiticcapacitanceCgs2020/5/2031DatalinePixelelectrodeEquivalentCircuitforpixelandcurveofcharginganddischarging2020/5/2032FeedthroughVoltage2020/5/2033GateSignal2020/5/2034Chargingtime2020/5/2035standardRatioofimageDefinition(column*rowNo.spotsNo.pixelsChoicetimeforscanline(us)SourceSignal2020/5/2036VoltageforGrayScale2020/5/2037TTVLCGrayScaleAsimplemodelofpixelcircuits2020/5/2038DischargingProcess2020/5/2039HoldingCurves2020/5/2040Summaryforpixelcircuit2020/5/2041CsonGate2020/5/2042DatalinescanlineCsscanlinepixelgatelineadjacentgatelineTFTstoragecapacitorPixelconfigurationwithstoragecapacitorInversionMode2020/5/2044InversionMode(cont.)2020/5/2045MeritsofDotInversion2020/5/2046LowFlickerLowCrosstalkCrosstalk2020/5/2047NormalPictureHorizontalCrosstalkVerticalCrosstalkSummaryforTFTdriver2020/5/2048RequirementforTFT2020/5/2049Ion/IoffofTFT3～4×106IleakageofTFT10-12AVoltagedifferenceforswitchofTFTisnottoolargeExcellentuniformityforTFTproperty3TFTtechnology1.TFTfabrication2.amorphousvs.crystallineSi3.Topvs.BottomgateTFT•Electrodes(ITO,Mo,Cr,Al)aresputtered•ChannellayerisPECVD-deposited•InsulatorlayerisPECVD-deposited•Structuringoflayersvialithographyanddry/wetetchgatesourcedrainglasssubstrate(T350oC!)channelinsulatorTFTfabricationTFTfabricationissimilartosiliconICprocessesGateline(rows)Sourcelines(columns)StoragecapacitorTFTTransmissivepixelconfigurationGatelineSourcelinePixelTFTin3DCross-overWhatisLTPoly-Si?LiquidphasecrystallisationofSiusinganexcimerlaserShortthermaldiffusionlengthsothesubstrateremainsatlowtemperatures(LT)largearea,lowcostglasssubstratesExcimerLaser(PulsedUV)CondensinglensHomogeniserAttenuatorfe=?251EcEvEcEvLog(#electronstates)Log(#electronstates)DeepstatesTailstatesCrystallinesiliconAmorphoussiliconsilisilicon•atomsareorderedincrystallattice•n