Semiempirical expression for direct transconductance and equivalent saturated velocity in short-gate-length MESFETs

A very simple and straightforward relationship has been derived between the direct transconductance and the apparent electron saturation drift velocity in GaAs MESFETs. From a comparison with experimental data it was found that this velocity may exceed the usual equilibrium velocity value. A new semiempirical relationship between the apparent non equilibrium velocity and the gate length is suggested. This relationship provides good agreement with both published experimental data and published Monte-Carlo numerical simulations. The practical implications on the saturated current at a given gate voltage, and on the maximum available transconductance per unit gate width for a given gate length and a given epilayer thickness are outlined. Moreover, the validity of present results for ion-implanted MESFETs is tested.