Lecture #38.ppt

Lecture#heMOSFET:Bulk-chargetheoryBodyeffectparameterChannellengthmodulationparameterPMOSFETI-VSmall-signalmodelReading:FinishChapter17,*Problemwiththe“SquareLawTheory”Ignoresvariationindepletionwidthwithdistancey2SemiconductorDevicesforIntegratedCircuits*Modified(Bulk-Charge)Modelsaturationregion:linearregion:3SemiconductorDevicesforIntegratedCircuits*TheexpressionthatwaspreviouslyderivedforVTisthegatevoltagereferencedtothebodyvoltagethatisrequiredreachthethresholdcondition:MOSFETThresholdVoltage,VTUsually,,andtheequationsforIDSare4SemiconductorDevicesforIntegratedCircuits*NotethatVTisafunctionofVSB:TheBodyEffectwheregisthebodyeffectparameterWhenthesource-bodypnjunctionisreverse-biased,|VT|,wewanttominimizegsothatIDsatwillbethesameforalltransistorsinacircuit5SemiconductorDevicesforIntegratedCircuits*,usingalowvalueofVDSIDSVGS6SemiconductorDevicesforIntegratedCircuits*ChannelLengthModulationParameter,lRecallthatasVDSisincreasedaboveVDsat,thewidthDLofthedepletionregionbetweenthepinch-offpointandthedrainincreases,*P-ChannelMOSFETThePMOSFETturnsonwhenVGS<VTpHolesflowfromSOURCEtoDRAINDRAINisbiasedatalowerpotentialthantheSOURCEInCMOStechnology,thethresholdvoltagesareusuallysymmetric:VTp=-VTnP+P+NGATEVSVDVGIDSVBVDS<0IDS<0|IDS|increaseswith|VGS-VTp||VDS|(linearregion)8SemiconductorDevicesforIntegratedCircuits*PMOSFETI-VLinearregion:Saturationregion:m=1+(3Toxe/WT)isthebulk-chargefactor9SemiconductorDevicesforIntegratedCircuits*SmallSignalModelConductanceparameters:10SemiconductorDevicesforIntegratedCircuits*