We present a comparative study of suppressing AlGaN/GaN HEMTs’ current collapse by two types of passivation materials: amorphous SiN_x (grown by PECVD and LPCVD) and epitaxial AlN (grown by plasma enhanced atomic layer deposition (PEALD). The motivation of this study stems from the different mechanisms underlying the two passivation techniques, namely, surface-charge-compensation by shallow donors (with SiN_x) and polarization charge (with epitaxial AlN). Since de-trapping from the shallow donor levels is a time-dependent process and the polarization charge compensation is a time-indepdent phenomenon, AlN-passivated AlGaN/GaN HEMTs was found to exhibit improved dynamic switching behavior than SiN_x-passivated devices. The results show that the depletion region is much narrower in AlN-passivated devices than SiN_x-passivated and un-passivated devices. The transient characteristics reveal the superiority of AlN passivation as the formation of depletion region is predominantly a field effect while the de-trapping from the shallow donor levels in SiN_x-passivated devices is limited by their intrinsic time constant. AlN-passivated devices show much smaller dynamic R_ON variation under both fast and slow switching operations.