BSIM3v3.2.2 MOSFET Model - The University of Texas at Dallas

Charge-Thickness Capacitance Modelcharges in the saturation region to the source electrode. Notice that thischarge partitioning scheme will still give drain current spikes in the linearregion and aggravate the source current spike problem.⎛( )Q W L C V A V A V2⎜gstefcvf bulk cveff bulk cveffs=− ⎜gsteff,c ''active active ox+ −2 4 ⎛ Abulk'⎜24⎜Vgsteff,cgsteffcv− V⎝⎝ 2cveff(4.3.25)⎞⎟⎟⎞⎟⎠⎟⎠⎛⎜Q W L C V A V A Vd=− ⎜gsteff,c3 'active active ox− +2 4 ⎛ A⎜8⎜Vgsteffcv−⎝⎝gsteff,c2( ' )gsteffcv bulk cveff bulk cveffbulk2'V(4.3.26)cveff⎞⎟⎟⎞⎟⎠⎟⎠(d) Bias-dependent threshold voltage effects on capacitanceConsistent Vth between DC and CV is important for acurate circuitsimulation. capMod=1, 2 and 3 use the same Vth as in the DC model.Therefore, those effects, such as body bias, DIBL and short-channel effectsare all explicitly considered in capacitance modeling. In deriving thecapacitances additional differentiations are needed to account for thedependence of threshold voltage on drain and substrate biases.4.4 Charge-Thickness Capacitance ModelCurrent MOSFET models in SPICE generally overestimate the intrinsiccapacitance and usually are not smooth at V fb and V th . The discrepancy is more4-14 BSIM3v3.2.2 Manual Copyright © 1999 UC Berkeley