Al Ga N/Ga N high electron-mobility transistors(HEMTs) with 5 nm Al N passivation by plasma enhanced atomic layer deposition(PEALD) were fabricated, covered by 50 nm Si Nx which was grown by plasma enhanced chemical vapor deposition(PECVD). With PEALD Al N passivation, current collapse was suppressed more effectively and the devices show better subthreshold characteristics. Moreover, the insertion of Al N increased the RF transconductance, which lead to a higher cut-off frequency. Temperature dependence of DC characteristics demonstrated that the degradations of drain current and maximum transconductance at elevated temperatures for the Al N/Si Nx passivated devices were much smaller compared with the devices with Si Nx passivation, indicating that PEALD Al N passivation can improve the high temperature operation of the Al Ga N/Ga N HEMTs. Al Ga N / Ga N high electron-mobility transistors (HEMTs) with 5 nm Al N passivation by plasma enhanced atomic layer deposition (PEALD) were fabricated, covered by 50 nm Si Nx which was grown by plasma enhanced chemical vapor deposition (PECVD) . With PEALD Al N passivation, current collapse was suppressed more effectively and the devices show better subthreshold characteristics. Furthermore, the insertion of Al N increased the RF transconductance, which lead to a higher cut-off frequency. the degradations of drain current and maximum transconductance at elevated temperatures for the Al N / Si Nx passivated devices were much smaller compared with the devices with Si Nx passivation, indicating that PEALD Al N passivation can improve the high temperature operation of the Al Ga N / Ga N HEMTs.