Problems in Precision Modeling of the MOS Transistor for Analog Applications

This paper summarizes the inadequacies of present MOSFET models as applied to analog circuit design and, in some cases, proposes solutions. Both efficient models suitable for CAD and more complex models are considered. Problem areas discussed include poor modeling of the moderate inversion region, poor modeling of the surface potential in strong inversion, ambiguous use of "threshold" voltages and poor expressions for them, poor modeling of the drain small-signal conductance, very poor modeling of intrinsic small-signal capacitances, inadequate small-signal equivalent circuit topologies, and poor implementation of known correct ideas in some CAD programs, including the dependence of the effective mobility on the substrate potential, the modeling of thermal noise in the nonsaturation region, and the modeling of ion-implanted devices.