SOURCE-DRAIN BURNOUT MECHANISM OF GAAS POWER MESFETS - 3 TERMINAL EFFECTS

Theoretical expressions for thermal and electrical feedback effects ar e derived. These limit the power capability of a power FET and lead a device to catastrophic breakdown (source-drain burnout) when the loop gain of the former reaches unity. Field emission of thermally excited electrons at the Schottky gate plays the key role in thermal feedback, while holes being impact ionized by the drain current play a similar role in the electrical feedback. Thermal feedback is dominant in a hig h temperature and low drain voltage area. Electrical feedback is domin ant in a high drain voltage and low temperature area. In the first are a, a high junction temperature is the main factor causing the thermal runaway of the device. In the second area, the electrical feedback inc reases the drain current and the temperature and gives a trigger to th e thermal feedback so that it reaches unity more easily. Both effects become significant in proportion to transconductance and gate bias res istance, and cause simultaneous runaway of the gate and drain currents . The expressions of the loop gains clearly indicate the safe operatin g conditions for a power FET. C-band 4 W (1 chip) and 16 W (4 chip) Ga As MESFETs were used as the experimental samples. With these devices t he simultaneous runaway of the gate and the drain currents, apparent d ependence of the three terminal breakdown voltage on the gate bias res istance in the region dominated by electrical feedback, the rapid incr ease of the field emitted current at the critical temperature and clea r coincidence between the measured and calculated three terminal gate currents both in the thermal feedback dominant region, etc, are demons trated. The theory explains the experimental results well. (C) 1997 El sevier Science Ltd.