extrapolated threshold voltage (V_T(ext))

The threshold voltage extrapolated from measurement of the maximum slope g_m(max) of the I_D-V_GS curve, as described in ASTM F617‑86. V_T(ext) can be calculated using

V_T(ext) = V_GS(gm(max)) - I_D(gm(max)) /g_m(max)

where

V_GS(gm(max)) is the gate voltage at the point of the maximum slope of the I_D-V_GS curve;
I_D(gm(max)) is the drain current at the point of the maximum slope of the I_D-V_GS curve;
g_m(max) is the maximum slope of the I_D-V_GS curve in the linear region.

References