Lecture #36Qualitative Theory of the NMOSFETSlide 3Ideal MOSFET I-V CharacteristicsImpact of Inversion-Layer BiasEffect of VCB on fS, W, and VTNMOSFET I-V Characteristics1st-Order ApproximationNMOSFET Current (1st-order approx.)Saturation Current, IDsatEE130 Lecture 36, Slide 1Spring 2007Lecture #36ANNOUNCEMENTS•Updated information for Term Project was posted on 4/14•Reminder: Coffee Hour today at ~4PM!OUTLINE The MOSFET:•Qualitative theory•Long-channel I-V (“Square-Law” Theory)Reading: Textbook Chapter 17.2, 18.3.4EE130 Lecture 36, Slide 2Spring 2007Qualitative Theory of the NMOSFETdepletion layerThe potential barrier to electron flow from the source into the channel is lowered by applying VGS> VTElectrons flow from the source to the drain by drift, when VDS>0. (IDS > 0.)The channel potential varies from VS at the source end to VD at the drain end.(The inversion layer can be modeled as a resistor.)VGS < VT:VGS > VT :VDS 0VDS > 0LVWQWQvWQIDSeffinveffinvinvDSEE130 Lecture 36, Slide 3Spring 2007When VD is increased to be equal to VG-VT, the inversion-layer charge density at the drain end of the channel equals zero, i.e. the channel becomes “pinched off”As VD is increased above VG-VT, the length L of the “pinch-off” region increases. The voltage applied across the inversion layer is always VDsat=VGS-VT, and so the current saturates:If L is significant compared to L, then IDS will increase slightly with increasing VDS>VDsat, due to “channel-length modulation”DsatDSVVDSDsatIIVGS > VT :VDS = VGS-VTVDS > VGS-VTEE130 Lecture 36, Slide 4Spring 2007Ideal MOSFET I-V CharacteristicsLinearregionSaturationregion(Enhancement Mode NMOS Transistor)EE130 Lecture 36, Slide 5Spring 2007Impact of Inversion-Layer Bias•When a MOS device is biased into inversion, a pn junction exists between the surface and the bulk.•If the inversion layer contacts a heavily doped region of the same type, it is possible to apply a bias to this pn junction.N+ poly-Sip-type Si------+ + + + + +N++ +-- -SiO2• VG is biased so that surface is inverted• n-type inversion layer is contacted by N+ region• If a bias VC is applied to the channel, A reverse bias (VB-VC) is applied between the channel & bodyEE130 Lecture 36, Slide 6Spring 2007Effect of VCB on S, W, and VT•Application of a reverse body bias non-equilibrium–2 Fermi levels (one for n-region, one for p-region)•separation = qVBC S is increased by VCB•Reverse body bias widens W, increases QdepQinv decreases with increasing VCB, for a given VGBoxCBFSiAFCBFBTCVqNVVV)2(22EE130 Lecture 36, Slide 7Spring 2007NMOSFET I-V Characteristics•VD > VS•Current in the channel flows by drift•Channel voltage VC(y) varies continuously between the source and the drain•Channel inversion charge densityoxedepSCBFBGoxeinvCyQyVVVCyQ)(2)()(oxCBFSiAFCBFBTCyVqNyVVV))(2(22)(WEE130 Lecture 36, Slide 8Spring 20071st-Order Approximation•If we neglect the variation of Qdep with y, thenwhere VT = threshold voltage at the source end: CSTGoxeinvCBSBTGoxeinvSBFSiAdepVVVVCQVVVVCQVqNQ )2(2oxSBFSiAFSBFBTCVqNVVV)2(22EE130 Lecture 36, Slide 9Spring 2007NMOSFET Current (1st-order approx.)•Consider an incremental length dy in the channel. The voltage drop across this region isDSDSTGoxeeffDSVVCCinveffDSVVCCinveffLDSeffinvDSinveffDSinvDSDSCVVVVCLWIdVVQLWIdVVWQdyIWQdyInWTqdyIWTdyIdRIdVDSDS2)()(0in the linear region2)(2TGeffoxeDsatDSVVCLWII in the saturation regionEE130 Lecture 36, Slide 10Spring 20072)(2TGeffoxeDsatVVCLWI • saturation region:Saturation Current, IDsatTGDsatDVVVV
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