7.8 Switch Unified (SU) MOSFET Model

• This section presents a more elaborate model of the MOSFET and can be skipped without loss of continuity.
• For a fixed VGS, we approximated the behavior as a linear resistor using the SR model.
• Clearly, the SR model does not capture MOSFET behavior if we vary VGS.
• Worse yet, even for a given value of VGS, the SR model becomes inaccurate as the value of VDS approaches VGS − VT.
• For more accuracy, we can develop a more elaborate model for the triode region operation of the MOSFET.
• Abandoning the piecewise-linear method, this more elaborate model characterizes the behavior of the MOSFET in the triode region as a nonlinear resistor, whose characteristics depend on VGS.
• When combined with the SCS model for the saturation region, the nonlinear resistor model in the triode region results in a continuous set of MOSFET curves.
• The resulting combined model for the triode and saturation regions is called the switch unified model or the SU model of the MOSFET.