A small-signal analytical theory for GaAs field-effect transistors at large drain voltages

Small-signal microwave performance of GaAs field-effect transistors (FET's) at large drain voltages is investigated, using a new analytical model which takes into account the carrier drift-velocity reduction and saturation due to electron upper valley scattering, and the extension of the depletion layer towards a drain side. Both of these play important roles in FET operation at large drain voltages. Small-signal y-parameters are calculated and the transit time effect which occurs in high-frequency operations is shown explicitly. Equivalent FET circuit elements are derived from the obtained y-parameters. Their dependence on device physical parameters, as well as on dc bias conditions, is calculated. The theoretical results are compared with the measured small-signal characteristics of a practical power GaAs FET and a reasonable agreement between them is obtained.