Physical modeling of microwave transconductance and capacitances in GaAs MESFETs operated in velocity saturation

Described is an analytical model applicable at high drain voltage which yields simple equations for the key small-signal equivalent circuit elements describing the microwave performance of GaAs MESFETs. The depletion region resulting from the gate potential is represented in an idealized shape. Use is made of Gauss´s law to obtain the dimensions of this shape. The response of this to applied signals is postulated to represent that within the MESFET to derive the equivalent circuit values of transconductance, input, and feedback capacitance. The behavior of the gate-depletion extension towards the drain, a function of the gate recess surface potential, and applied potentials, is shown to predict the drain bias dependence. The performance of the FET is described in terms of the FETs physical structure; the equations therefore provide valuable tools for process control and sensitivity analysis