Numerically Robust $S$-Domain Resistance Extraction for Equivalent-Circuit Modeling of Microwave FETs

A direct S-domain approach for extracting extrinsic and intrinsic resistances of microwave FETs under active and cold pinch-off conditions, respectively, is presented. The classical Z-domain extraction is shown to be numerically fragile when reactive contributions dominate. The proposed method exploits constant-series resistance circles in the Smith chart and an overdetermined formulation to obtain physically consistent resistance values without passing to the impedance representation. The approach is validated both through Monte Carlo analysis and measurements on GaAs pHEMT devices, demonstrating improved robustness against frequency range selection and residual capacitive deembedding errors.