Unexpected Losses in GaN HEMTs have limited their performance at high frequencies, one of the most relevant are the unwanted hysteresis when we charge and discharge the output capacitance that can lead at large power losses, decreasing the efficiency of the device. This phenomenon is still unclear but the most accredited hypothesis concerns defects in the GaN Buffer that trap the high-energy electrons, causing an instability of charges during one cycle and so generating the hysteresis. The most used technique for the evaluation of Output Capacitance Losses is the Sawyer-Tower method, but its limitations due to the reference series capacitor connection and temperature dependace do not allow a complete understanding of the effect.