We have studied the degradation mechanism of Al- GaAdInGaAs pseudomorphic HEMTs (FHEMTs) under high humidity conditions. The degraded samples under high humidity condition 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, which indicates that Vth shft is caused by piezoelectric effect due to stress change near the gate. Cross-sectional transmission electron microscopy (TEM) images from the deteriorated devices reveal the existence of a damaged tecess surface region and a peeling of passivation film. At the interface between passivation fh and AlGaAs surface, diffusion of Ga, As and AI into passivation film was observed by secondary ion mass spectrometry (SIMS). From these results, degradation of PHEMT has mainly two mechanisms: positive shift in Vth due to stress change under the gate which might be caused by the peeling of passsivation, and a decrease in Imax due to surface degradation at AlGaAs recess regions caused by diffusion phenomena of Ga,As and AI. The pre-deposition treatment effectively suppresses the degradation of pHEMTs under high humidity without degradation of high frequency performances.
A 19 GHz extremely low phase noise voltage controlled oscillator (VCO) MMIC is presented. To reduce the phase noise of the VCO, a hetemstructure field effect transistor (HFET) is used as the active device, because its property of low frequency noise is superior to that of high electron mobility transistor (HEMT). This VCO showed the typical phase noise of -120 dBdHz at 1 MHz offset from carrier. This performance is better than other VCOs operating above 10 GHz. Measured tuning range is 400 MHz and output power is 2 dBm. The fabricated MMIC chip size is 2.7 mm x 1.4 mm.0-7803-7695-1/03/
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