Off-State Breakdown in GaAs Power HFETs

This work brings new experimental data shedding light on the still controversial issue of off-state breakdown in microwave power FETs, which limits the power handling capability. By means of measurements performed on AlGaAs/GaAs power HFETs in a wide range of temperatures, we show that: 1) At relatively low drain-gate voltage, the gate current comes from thermionic-field emission. .2) As we approach off-state breakdown, impact ionization of electrons accelerated by the large drain-gate field becomes the dominant mechanism originating the gate current. 3) For even higher values of the drain-gate volt- age, both electrons and holes possibly ionize, and the drain-gate diode avalanches. Our experimental findings diverge in some respects from other published data and represent therefore a new contribution to the understanding of this important issue.