Lecture 17The Body EffectBody Effect ExampleChannel-Length Modulation and LVelocity SaturationImpact of Velocity SaturationShort-Channel MOSFET ID-VDSDrain Induced Barrier Lowering (DIBL)NMOSFET in OFF StateSub-Threshold Leakage CurrentShort-Channel MOSFET ID-VGSVTH Design Trade-OffMOSFET Large-Signal Models (VGS > VTH)MOSFET Transconductance, gmMOSFET Small-Signal Model (Saturation Region of Operation)Derivation of Small-Signal Model (Long-Channel MOSFET, Saturation Region)PMOS TransistorPMOS I-V EquationsCMOS TechnologyComparison of BJT and MOSFETEE105 Fall 2007 Lecture 17, Slide 1 Prof. Liu, UC BerkeleyLecture 17OUTLINE•NMOSFET in ON state (cont’d)–Body effect–Channel-length modulation–Velocity saturation•NMOSFET in OFF state•MOSFET models•PMOSFETReading: Finish Chapter 6ANNOUNCEMENTS•Wed. discussion section moved (again) to 6-7PM in 293 CoryEE105 Fall 2007 Lecture 17, Slide 2 Prof. Liu, UC BerkeleyThe Body Effect BSBBTHBSBBoxSiATHoxSBBSiAoxBSiAoxBSiABFBoxSBBSiABFBTHVVVCqNVCVqNCqNCqNVCVqNVV22222)2(2)2(2)2(22)2(2200•VTH is increased by reverse-biasing the body-source PN junction: is the body effect parameter.EE105 Fall 2007 Lecture 17, Slide 3 Prof. Liu, UC BerkeleyBody Effect Example oxSiABSBBTHTHCqNVVV2 where220EE105 Fall 2007 Lecture 17, Slide 4 Prof. Liu, UC BerkeleyChannel-Length Modulation•The pinch-off point moves toward the source as VDS increases.The length of the inversion-layer channel becomes shorter with increasing VDS.ID increases (slightly) with increasing VDS in the saturation region of operation. satDDSTHGSoxnsatDVVVVLWCI,2,121 is the channel length modulation coefficient.LLLLLIDsat111DSsatDSVVL EE105 Fall 2007 Lecture 17, Slide 5 Prof. Liu, UC Berkeley and L•The effect of channel-length modulation is less for a long-channel MOSFET than for a short-channel MOSFET.EE105 Fall 2007 Lecture 17, Slide 6 Prof. Liu, UC BerkeleyVelocity Saturation•In state-of-the-art MOSFETs, the channel is very short (<0.1m); hence the lateral electric field is very high and carrier drift velocities can reach their saturation levels.–The electric field magnitude at which the carrier velocity saturates is Esat.Siin holesfor cm/s 106Siin sonfor electr cm/s 10866satvvEEE105 Fall 2007 Lecture 17, Slide 7 Prof. Liu, UC BerkeleyImpact of Velocity Saturation•Recall that•If VDS > Esat×L, the carrier velocity will saturate and hence the drain current will saturate:• ID,sat is proportional to VGS–VTH rather than (VGS – VTH)2• ID,sat is not dependent on L• ID,sat is dependent on W)()( yvyWQIinvD satTHGSoxsatinvsatDvVVWCvWQI ,EE105 Fall 2007 Lecture 17, Slide 8 Prof. Liu, UC Berkeley•ID,sat is proportional to VGS-VTH rather than (VGS-VTH)2• VD,sat is smaller than VGS-VTH• Channel-length modulation is apparent (?)Short-Channel MOSFET ID-VDSP. Bai et al. (Intel Corp.), Int’l Electron Devices Meeting, 2004.EE105 Fall 2007 Lecture 17, Slide 9 Prof. Liu, UC Berkeley•In a short-channel MOSFET, the source & drain regions each “support” a significant fraction of the total channel depletion charge Qdep×W×L VTH is lower than for a long-channel MOSFET•As the drain voltage increases, the reverse bias on the body-drain PN junction increases, and hence the drain depletion region widens.VTH decreases with increasing drain bias.(The barrier to carrier diffusion from the source into the channel is reduced.) ID increases with increasing drain bias.Drain Induced Barrier Lowering (DIBL)EE105 Fall 2007 Lecture 17, Slide 10 Prof. Liu, UC BerkeleyNMOSFET in OFF State•We had previously assumed that there is no channel current when VGS < VTH. This is incorrect!•As VGS is reduced (toward 0 V) below VTH, the potential barrier to carrier diffusion from the source into the channel is increased. ID becomes limited by carrier diffusion into the channel, rather than by carrier drift through the channel.(This is similar to the case of a PN junction diode!)ID varies exponentially with the potential barrier height at the source, which varies directly with the channel potential.EE105 Fall 2007 Lecture 17, Slide 11 Prof. Liu, UC BerkeleySub-Threshold Leakage Current•Recall that, in the depletion (sub-threshold) region of operation, the channel potential is capacitively coupled to the gate potential. A change in gate voltage (VGS) results in a change in channel voltage (VCS):•Therefore, the sub-threshold current (ID,subth) decreases exponentially with linearly decreasing VGS/mmVCCCVVGSdepoxoxGSCS/log (ID)VGSIDVGSmV/dec60)10(ln)(log110TGSDSmVSdVIdS“Sub-threshold swing”:EE105 Fall 2007 Lecture 17, Slide 12 Prof. Liu, UC BerkeleyShort-Channel MOSFET ID-VGSP. Bai et al. (Intel Corp.), Int’l Electron Devices Meeting, 2004.EE105 Fall 2007 Lecture 17, Slide 13 Prof. Liu, UC BerkeleyVTH Design Trade-Off•Low VTH is desirable for high ON-state current:ID,sat (VDD - VTH) 1 < < 2•But high VTH is needed for low OFF-state current:VTH cannot be reduced aggressively.Low VTHHigh VTHIOFF,high VTHIOFF,low VTHVGSlog ID0EE105 Fall 2007 Lecture 17, Slide 14 Prof. Liu, UC BerkeleyMOSFET Large-Signal Models (VGS > VTH)•Depending on the value of VDS, the MOSFET can be represented with different large-signal models. satDDSTHGSoxsatsatDsatDDSTHGSoxnsatDVVVVWCvIorVVVVLWCI,,,2,1)(121VDS << 2(VGS-VTH))(1THGSoxnONVVLWCRVDS < VD,satDSDSTHGSoxntriDVVVVLWCI2)(,VDS > VD,satEE105 Fall 2007 Lecture 17, Slide 15 Prof. Liu, UC BerkeleyMOSFET Transconductance, gm•Transconductance (gm) is a measure of how much the drain current changes when the gate voltage changes.•For amplifier applications, the MOSFET is usually operating in the saturation region.–For a long-channel MOSFET:–For a short-channel MOSFET: DsatDDSoxnmsatDDSTHGSoxnmIVVLWCgVVVVLWCg,,121GSDmVIg satDDSoxsatmVVWCvg,1 EE105 Fall 2007 Lecture 17, Slide 16 Prof. Liu, UC BerkeleyMOSFET Small-Signal Model
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