Handout #5EE 214Winter 2009MOS Transistor Modeling for Analog DesignPart IIB. Murmann and B. A. WooleyStanford UniversityCorrections:1/23/09: Fixed ωp2expression on slide 32; see also text p. 641Re-capSubthresholdOperationTransition to ?Strong InversionWhat causes the discrepancy between 2/VOVand 0.18μm NMOS in strong inversion?B. A. Wooley, B. Murmann EE214 Winter 2008-092
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Short Channel EffectsVelocity saturation due to high lateral fieldMobility degradation due to high vertical fieldVtdependence on channel length and widthVt= f(VDS)ro= f(VDS)…We will limit the discussion in EE214 to the first two aspects of the above list, with a focus on qualitative understandingB. A. Wooley, B. Murmann EE214 Winter 2008-093Velocity Saturation (1)Ithditif thld l it id th t thiIn the derivation of the square law model, it is assumed that the carrier velocity is proportional to the lateral E-field, v=μEUnfortunately, the speed of carriers in silicon is limited–At very high fields (high voltage drop across the conductive channel), the carrier velocity saturatesApproximation:ccμEν(E) μEforEE1dsclvE=≅=>>cE+12dcsclv (E )v=B. A. Wooley, B. Murmann EE214 Winter 2008-094