Extract Subthreshold Slope
The problem will be deciding which region is a good fit for the subtheshold slope as the slope remains constant and then transitions to a &quot;second order&quot; polynomial.
Do a first order polynomial fit of Vg vs Id (of only the subtheshold region), the polynomial fit will yield a  number for slope and a number for offset. The number for slope is the subtheshold slope.
Finding the differential also could be useful, after doing so the subtheshold region would be flat.