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Berkeley ELENG 105 - Lecture 17

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Lecture 17ANNOUNCEMENTS•Wed discussion section moved (again) to 6‐7PM in 293 CoryOUTLINE•Wed. discussion section moved (again) to 6‐7PM in 293 Cory• NMOSFET in ON state (cont’d)– Body effectChannellength modulation–Channel‐length modulation– Velocity saturation•NMOSFET in OFF stateNMOSFET in OFF state• MOSFET models•PMOSFETEE105 Fall 2007 Lecture 17, Slide 1Prof. Liu, UC BerkeleyPMOSFETReading: Finish Chapter 6The Body EffectVqNφε)2(2+• VTHis increased by reverse‐biasing the body‐source PN junction:SBBSiABSiABSiAoxSBBSiABFBTHVqNqNqNVCVqNVVφεφεφεφφεφ)2(2)2(2)2(22)2(22+++++++=()()BSBBTHBSBBSiATHoxoxoxBFBVVVCqNVCCCVφφγφφεφ22222200−++=−++=+−++=EE105 Fall 2007 Lecture 17, Slide 2Prof. Liu, UC Berkeley()()BSBBTHBSBBoxTHCφφγφφ00γ is the body effect parameter.Body Effect Example()BSBBTHTHNVVVφφγ2220−++=oxSiACqNεγ2 where =EE105 Fall 2007 Lecture 17, Slide 3Prof. Liu, UC BerkeleyChannel‐Length Modulation• The pinch‐off point moves toward the source as VDSincreases.Æ The length of the inversion‐layer channel becomes shorter with increasing VDS.Æ IDincreases (slightly) with increasing VDSin the saturation region of operation.DSsatDSVVL−∝∆⎟⎠⎞⎜⎝⎛∆+≅∆∝LLLLLIDsat111⎠⎝∆−LLLL()()[]satDDSTHGSoxnsatDVVVVLWCI,2,121−+−=λµEE105 Fall 2007 Lecture 17, Slide 4Prof. Liu, UC Berkeleyλis the channel length modulation coefficient.λand L• The eff ect of channel‐length modulation is less for a long‐channel MOSFET than for a short‐channel MOSFET.EE105 Fall 2007 Lecture 17, Slide 5Prof. Liu, UC BerkeleyVelocity Saturation• In state‐of‐the‐art MOSFETs, the channel is very short (<0.1µm); hence the lateral electric field is very high and carrier drift y gvelocities can reach their saturation levels.– The electric field magnitude at which the carrier velocity saturates is Esat.⎩⎨⎧××=Siin holesfor cm/s106Siin sonfor electr cm/s 10866satvv⎩EEE105 Fall 2007 Lecture 17, Slide 6Prof. Liu, UC BerkeleyImpact of Velocity Saturation• Recall that)()( yvyWQIinvD=• If VDS> Esat×L, the carrier velocity will saturate and hence the drain current will saturate:()satTHGSoxsatinvsatDvVVWCvWQI−==,• ID,satis proportional to VGS–VTHrather than (VGS–VTH)2• ID,satis not dependent on L• ID,satis dependent on WEE105 Fall 2007 Lecture 17, Slide 7Prof. Liu, UC BerkeleyShort‐Channel MOSFET ID‐VDSP. Bai et al. (Intel Corp.), Int’l Electron Devices Meeting 2004Int’l Electron Devices Meeting, 2004.• ID,satis proportional to VGS‐VTHrather than (VGS‐VTH)2• VD,satis smaller than VGS‐VTHEE105 Fall 2007 Lecture 17, Slide 8Prof. Liu, UC Berkeley,satGS• Channel‐length modulation is apparent (?)Drain Induced Barrier Lowering (DIBL)• In a short‐channel MOSFET, the source & drain regions each “support” a significant fraction of the total channel depletion charge Qdep×W×LpÆ VTHis 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 widensjunction increases, and hence the drain depletion region widens.ÆVTHdecreases with increasing drain bias.(The barrier to carrier diffusion from the source into the channel is reduced.)EE105 Fall 2007 Lecture 17, Slide 9Prof. Liu, UC BerkeleyÆ IDincreases with increasing drain bias.NMOSFET in OFF State• We had previously assumed that there is no channel current whenVGS<VTH. This is incorrect!when VGS VTH. This is incorrect!• As VGSis reduced (toward 0 V) below VTH, the potential barrier to carrier diffusion from the source into the channel is increased. b li i d b i diff ii h hlhIDbecomes 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!)( j )ÆIDvaries exponentially with the potential barrier height at the source, which varies directly with the channel potential.EE105 Fall 2007 Lecture 17, Slide 10 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. p pyp g pA change in gate voltage (∆VGS) results in a change in channel voltage (∆VCS):C⎞⎜⎛•Therefore the sub‐threshold current (IDbh) decreasesmVCCCVVGSdepoxoxGSCS/∆≡⎟⎠⎜⎜⎝+×∆=∆Therefore, the subthreshold current (ID,subth) decreases exponentially with linearly decreasing VGS/mlog (ID)ID“Sub-threshold swing”:V/d60)10(l)(log110⎟⎟⎠⎞⎜⎜⎝⎛≡−GSDSVSdVIdSEE105 Fall 2007 Lecture 17, Slide 11 Prof. Liu, UC BerkeleyVGSVGSmV/dec60)10(ln >=TmVSShort‐Channel MOSFET ID‐VGSP. Bai et al. (Intel Corp.), Int’l Electron Devices Meeting, 2004.EE105 Fall 2007 Lecture 17, Slide 12 Prof. Liu, UC BerkeleyVTHDesign Trade‐Off• Low VTHis desirable for high ON‐state current:I(VV)12ID,sat∝(VDD‐VTH)η1 < η< 2•But highVis needed for low OFFstate current:•But high VTHis needed for low OFF‐state current:Low Vlog IDÆVTHcannot be reduced aggressively.Low VTHHigh VTHIOFF,low VTHDIOFF,high VTHOFF,low VTHEE105 Fall 2007 Lecture 17, Slide 13 Prof. Liu, UC BerkeleyVGS0MOSFET Large‐Signal Models (VGS> VTH)• Depending on the value of VDS, the MOSFET can be represented with different large‐signal models. ggVDS<< 2(VGS-VTH) VDS< VD,satVDS>


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Berkeley ELENG 105 - Lecture 17

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