Degradation mechanisms of GaAs PHEMTs in high humidity conditions

We have studied the degradation mechanism of AlGaAs/InGaAs pseudomorphic HEMTs (PHEMTs) under high humidity conditions. The samples show a decrease in maximum drain current (Imax) and positive shift in threshold voltage (Vth). It was found that the Vth shift depends on gate orientation, caused by a piezoelectric effect due to stress change near the gate. Cross-sectional TEM images from the deteriorated devices reveal the existence of a damaged recess surface region and a peeling of the passivation film (PF). At the interface between the PF and AlGaAs surface, diffusion of Ga, As and Al into the PF was observed by SIMS. From these results, degradation of the PHEMT has two main mechanisms: positive shift in Vth due to stress change under the gate which might be caused by the peeling of the PF, and a decrease in Imax due to surface degradation at AlGaAs recess regions caused by diffusion phenomena of Ga,As and Al. The pre-deposition treatment effectively suppresses the degradation of PHEMTs under high humidity without degradation of high frequency performance.