The measurement circuit is as in the following from LTspice. Vg is stepped to obtain a range of drain current, while Vdd is adjust at each step to hold Vd1 at a given value, for example, 2 V. Node voltages are measured as well as VRd for current measurement. Also, at output characteristic is obtained for M1 with stepping Vdd for a given Vg1. KP, VTO, and LAMBDA, for a given NSUB, are obtained with M1 curve fits. The simulator is EKV v262.
A UCRIT sweep finds the best value for this parameter with a best fit to the output characteristic in pre-sat, as in this example. The Y axis in the plot is the mean absolute point difference, measured and simulated, in percent.
The drain current is then simulated as a function of the node voltages of M2, Vg2, Vd2, and Vs2. A result follows with an NSUB adjustment for a fit. Nsub = 1.6 x 10^22. (Meters.)
This is an example of the plots without the NSUB fit. (Nsub = 1.7 x 10^22)
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