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This paper summarizes the inadequacies of present MOSFET models as applied to analog circuit design. Both efficient models suitable for CAD and more complex models are considered. Problem areas discussed include poor modeling of the moderate inversion region, ambiguous use of "threshold" voltages, poor modeling of the drain small-signal conductance, and very poor modeling of intrinsic small-signal capacitances.
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Y. Tsividis (1982) studied this question.