半导体器件物理mosfet-part3课件

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1、LongchannelMOSFETsNowweunderstandthecharacteristicsoftheMOScapacitor.Forinstance,thesurfaceofap-Sisubstratecanbeinvertedtobecomentypeifthegatevoltageispositiveenough.LongchannelMOSFETsLongchannelMOSFETsLongchannelMOSFETsLongchannelMOSFETsLongchannelMOSFETsLongchannelMOSFETsTwoapproximati

2、onsforlongchannelMOSFETs:Gradualchannelapproximation(GCA)ChargesheetapproximationDrainCurrentModelCoordinates:x:normaltoSisurfacey:paralleltocurrentdirectionz:perpendiculartocurrentdirectionL:channellengthW:channelwidthTerminalvoltages:Vds:drainvoltage(relativetosource)Vg:gatevoltageV(y)

3、:voltageatpointyalongchannel(relativetosource)GradualChannelApproximationThevariationoftheelectricfieldalongthechannel(y-direction)<

4、icswhereμnistheelectronmobilityinthechannel(<

5、nlyn(x,y)remainsinsidetheintegral.DefineQitobethechargeperunitareaunderthegate:I-VcharacteristicsNotethatVisafunctionofyonly.I-VcharacteristicsMultiplyingbothsidesbydyandintegratefromy=0toLgives:CurrentcontinuitymeansthatIdsisaconstantindependentofy.Hence,finally,ChargeSheetApproximation

6、Theinversionchargesarelocatedexactlyatthesiliconsurfaceandformsasheetofchargewithzerothickness.Thereisnopotentialdropacrosstheinversionlayer.Note:ByCSA,ananalyticalsolutiontothedraincurrentcanbefound.I-VcharacteristicsThesurfacepotentialisψs=2ψB+V(y)becauseoftheonsetofstronginversion.Usi

7、ngthedepletionapproximation,thebulkdepletionchargedensityis:I-VcharacteristicsThetotalchargedensityinsiliconis:TheinversionchargedensityisthedifferenceofQiandQs:BysubstitutingthisintoIdsandintegrating,weget:I-VcharacteristicforalongchannelMOSFET.TworegionsofMOSFEToperatio