An approach including the effects of nonstationary electron dynamics is used to derive a small-signal equivalent circuit model useful for the evaluation of GaAs device structures. The calculated results show that (1) the nonstationary electron transport decreases not only the active channel resistance but also both the feedback resistance and the output impedance, so that instability effects are enhanced and the maximum stable gain is decreased; and that (2) the high field domain plays a major role for small-signal properties of sub-mum GaAs MESFETs