This paper presents reliability study of GaN HEMTs, emphasizing the application of S-parameter analysis to identify degradation mechanisms complementary to conventional DC metrics. By employing small-signal S-parameter analysis, we systematically differentiate degradation effects occurring in different regions of the device. We identify surface trapping, particularly at the drain edge of the gate under semi-on hot carrier stress, as the dominant degradation mechanism. A time-dependent recovery of the degradation is observed, suggesting against permanent hot-carrier degradation