Reliability of devices operating at premature on-state avalanche breakdown and a method using a proper electrical stress to alleviate the breakdown are investigated in deep submicron power AlGaAs/InGaAs PHEMT's. The results show that depending on the gate and drain biases in device stress, alleviation of premature on-state avalanche breakdown may be achieved without degrading PHEMT's DC and RF performance. On the other hand, PHEMT's may suffer catastrophic failure when stressed at Vds above the threshold value. This study facilitates a useful information for evaluating a reliability constraint imposed by the premature on-state avalanche breakdown and highlights a new direction to improving reliability and power performance of PHEMT's.