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A nonlinear III-V field-effect transistor model is proposed for designing microwave antenna switches and related circuits, off-state gate and drain capacitance values of a pseudo-morphic high-electron-mobility transistor (HEMT) derived from pulsed S-parameters with a deep OFF-state quiescent gate voltage are found to be much less voltage dependent than capacitance values measured by a nonpulsed bias owing to trap-induced dispersion effects. Our device model based on pulsed-bias CV characteristics accurately simulates switch nonlinearity. Both gate and drain capacitance values are assumed to be nonlinear, and a charge expression is developed for model implementation. For both capacitance values, a nonpulsed-bias CV curve is also utilized to maintain accurate capacitance at the quiescent voltage and, thus, accurate simulation of off-switch isolation. Additional terms are introduced to an existing drain current model to improve accuracy at high Vgs/low Vds and subthreshold regions. Furthermore, the model is extended to multiple-gate devices. Harmonics generated from both OFF- and ON-state switches, insertion loss, and isolation are accurately predicted for both single- and multiple-gate HEMTs.