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发表于 2011-11-16 15:36:56
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4.5 Extrinsic Capacitance: Overlap
Capacitance
An accurate model for the overlap capacitance is essential. This is especially true
for the drain side where the effect of the capacitance is amplified by the transistor
gain. In old capacitance models this capacitance is assumed to be bias
independent. However, experimental data show that the overlap capacitance
changes with gate to source and gate to drain biases. In a single drain structure or
the heavily doped S/D to gate overlap region in a LDD structure the bias
dependence is the result of depleting the surface of the source and drain regions.
Since the modulation is expected to be very small we can model this region with a
constant capacitance. However in LDD MOSFETs a substantial portion of the
LDD region can be depleted, both in the vertical and lateral directions. This can
lead to a large reduction of overlap capacitance. This LDD region can be in
accumulation or depletion. We use a single equation for both regions by using such
smoothing parameters as Vgs,overlap and Vgd,overlap for the source and drain side,
respectively. Unlike the case with the intrinsic capacitance, the overlap
capacitances are reciprocal. In other words, Cgs,overlap = Csg,overlap and Cgd,overlap
= Cdg,overlap
摘自BSIM3V3 manual |
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