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AlGaN/GaN HEMTs have achieved record output power densities at microwave frequencies; however, reliability of these devices is still a major concern. In this paper, the results of DC and RF stress tests at several drain voltages of passivated 75 μm devices are discussed. After DC stress for 48 hours, negligible differences in IN and small signal performance were observed in some devices, while significant reduction in drain current, decrease in transconductance, and increase in on-resistance were measured in some other devices. RF stress for 40 hours has resulted in lower transconductance and drain current and degradation in power performance at 10 GHz. A comprehensive comparison of equivalent circuit models before and after stress is presented.