Investigation of physical mechanisms limiting maximum output power and efficiency of MESFETs

It is shown for the first time that the maximum output power of MESFETs is limited by avalanche-injection instability, while maximum efficiency-by avalanche breakdown in the drain portion of Schottky barrier of the gate. Quasihydrodynamic equations have been solved numerically with the aid of an absolutely stable difference scheme offering stable solutions for mixed boundary conditions, i.e. with fixed electrode potentials and full electrode currents. The obtained numerical model fits current-voltage characteristics shown by V. Vashchenko. Our investigations have shown that three main leakage current sources exist in MESFETs and that maximum output power is limited by avalanche-injection instability. This instability is due to substrate leakage current in the built-in n/sup +/-i-n/sup +/ diode in parallel with an active transistor. The calculated maximum output power is about 1.5 W/mm, but the efficiency of such a transistor is quite small (about 7%) because of large leakage currents. By modifying the active output load and the working point we can tune the transistor to its maximum efficiency close to theoretical value of 50%; however, the output power decreases.