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

Methodology for Intrinsic Capacitance Modelingsaturation region is assumed to be zero. This is a good approximation forlong channel devices but fails when L eff < 2 µm. If we define a drain bias,V dsat,cv , in which the channel charge becomes a constant, we will find thatV dsat,cv in general is larger than V dsat but smaller than the long channel V dsat ,given by V gt /A bulk . However, in old long channel charge models, V dsat,cv isset to V gt /A bulk independent of channel length. Consequently, C ij /L eff has nochannel length dependence (Eqs. (4.3.6), (4.37)). A pseudo short channelmodification from the long channel has been used in the past. It involvedthe parameter A bulk in the capacitance model which was redefined to beequal to V gt /V dsat , thereby equating V dsat,cv and V dsat . This overestimated theeffect of velocity saturation and resulted in a smaller channel capacitance.The difficulty in developing a short channel model lies in calculating thecharge in the saturation region. Although current continuity stipulates thatthe charge density in the saturation region is almost constant, it is difficultto calculate accurately the length of the saturation region. Moreover, due tothe exponentially increasing lateral electric field, most of the charge in thesaturation region are not controlled by the gate electrode. However, onewould expect that the total charge in the channel will exponentiallydecrease with drain bias. Experimentally,VV,< V,< V,=active →∞Agsteff ,cvdsat iv dsat cv dsat iv Lbulk(4.3.9)and V dsat,cv is modeled by the following4-8 BSIM3v3.2.2 Manual Copyright © 1999 UC Berkeley