Degradation mechanisms of GaAs PHEMTs in high humidity conditions

We have studied the degradation mechanisms of AlGaAs/InGaAs pseudomorphic HEMTs (PHEMTs) under high humidity conditions (85 °C, 85% relative humidity). The degraded samples under high humidity conditions show a decrease in maximum drain current (Imax) and a positive shift in threshold voltage (Vth). Cross-sectional transmission electron microscopy (TEM) images from the deteriorated devices reveal an existence of damaged recess surface region and a peeling of a passivation film (SiNx). The secondary ion mass spectrometry (SIMS) depth profile at the interface between the passivation film and AlGaAs surface also indicates the diffusion of gallium (Ga), arsenic (As) and aluminum (Al) into the passivation film. The degradation of PHEMTs arises from mainly two mechanisms: (1) the positive shift in Vth due to stress change under the gate caused by the peeling of passivation films, and (2) the decrease in Imax due to the net carrier concentration reduction of the AlGaAs carrier supply layer caused by the combination of surface degradation at the AlGaAs recess regions and diffusion of Ga, As and Al at the interface between the passivation film and AlGaAs surface. A special treatment just prior to the deposition of SiNx films on the devices effectively suppresses the degradation of PHEMTs under high humidity conditions without degradation of the high frequency performance.