How do I extract a nonlinear transistor model from measured S-parameters and DC IV curves?

A nonlinear transistor model is constructed by combining DC I-V (current-voltage) measurements with multi-bias S-parameter measurements to capture both the static and dynamic behavior of the device: (1) DC I-V measurement: measure the drain current (I_DS) as a function of gate voltage (V_GS) and drain voltage (V_DS). This provides: the transconductance (g_m = dI_DS/dV_GS), the output conductance (g_ds = dI_DS/dV_DS), the threshold voltage (V_th), the knee voltage (V_knee), and the maximum current (I_DSS). These parameters define the nonlinear current source in the model (the core of the transistor model). (2) Multi-bias S-parameter measurement: measure the small-signal S-parameters at multiple bias points (varying V_GS and V_DS across the operating range). At each bias point: the S-parameters capture the linear behavior (gain, input/output impedance, feedback). From the S-parameters: extract the small-signal equivalent circuit elements at each bias: intrinsic elements (C_gs, C_gd, C_ds, g_m, g_ds, R_i, tau) and extrinsic elements (L_g, L_d, L_s, R_g, R_d, R_s, C_pg, C_pd). The extrinsic elements are bias-independent (they represent the physical parasitics of the device: pad capacitances, lead inductances, contact resistances). The intrinsic elements are bias-dependent (they represent the active device behavior). (3) Model construction: choose a model topology: Angelov (for GaN and GaAs HEMTs), Curtice (for GaAs FETs), or VBIC/HiCUM (for HBTs). Fit the nonlinear current source equations to the DC I-V data. Map the bias-dependent intrinsic elements (C_gs(V_GS, V_DS), C_gd(V_GS, V_DS), etc.) to the model charge equations. Fit the model parameters to reproduce the measured S-parameters across all bias points. (4) Verification: simulate the device with the extracted model and compare to: measured S-parameters at bias points NOT used in the extraction (interpolation test), measured large-signal data (load-pull, power sweep) if available, and harmonic behavior (second and third harmonic output vs input power).