The influence of the hot electron accelerated stress on DC characteristics of AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors (PHEMT's) is found to be related to the Schottky characteristics. The studies of reverse Schottky characteristics before and after stress are presented and found to be related to the following two major mechanisms: (1) the widening of the depletion under the gate after stress; (2) the influence of the carriers trapping under the gate after stress, which is mainly due to DX-centers. A new model based on the image force of Schottky barrier on hot electrons effects on leakage gate current is proposed. Both AlGaAs and InGaP PHEMT's with the extreme small variation of minimum noise figure and associated power gain measured at 12 GHz under hot electron accelerated stress will be investigated. Comparing the noise performance of AlGaAs PHEMT's with InGaP PHEMT's, the higher reliability in InGaP low noise PHEMT's will be demonstrated.