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The results of continuous wave short-term RF stress applied at 3 GHz on GaN high electron mobility transistors on silicon substrate are presented. The degradation of the device characteristics for RF overdrive conditions from 3-dB to 8-dB gain saturation is discussed. Output RF power degrades significantly in a short period of time. Both transient and permanent degradation of electronic properties of the device are identified. After high RF gain compression levels, DC characteristics like the threshold voltage and gate leakage current change permanently. Detailed microscopic changes in the electronic structure of the device were studied by performing simultaneous low-frequency noise measurements of gate and drain currents before and after stress. The channel was found to be immune to the whole stress regime with no increase of the Hooge parameter. On the other hand, activation of unstable defects and then an increase of the defect density near the gate metal semiconductor interface were observed from gate noise measurements. A point defect located at around 4.5 nm from the gate metal semiconductor interface with activation energy of 0.9 eV below the AlGaN conduction band edge was determined from random telegraph noise measurements. The role of forward gate biasing as a failure mechanism is also discussed.