AbstractMethods are discussed for forecasting, in a few seconds on a personal computer with a 486 processor, equivalent circuits and S‐parameters of a GaAs FET from a physical specification. By inverting the algorithms, similarly fast simulators can be devised to fit equivalent circuits to measured S‐parameters, in the process yielding information on the FET's material and structural dimensions. The practical value of such simulators is illustrated by FETs designed and implemented to have a noise figure and associated gain at 12 GHz comparable with commercial pseudomorphic HEMTs. © 1993 John Wiley&Sons,