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Passivation is commonly used to suppress current collapse in AlGaN/GaN HEMTs. However, the conventional PECV-fabricated SiNx passivation layer is incompatible with the latest process, like the "passivation-prior-to-ohmic" method. Research attention has therefore turned to high-temperature passivation schemes. In this paper, we systematically investigated the differences between the SiNx/GaN interface of two high-temperature passivation schemes, MOCVD-SiNx and LPCVD-SiNx, and investigated their effects on the ohmic contact mechanism. By characterizing the device interface using TEM, we reveal that during the process of MOCVD-SiNx, etching damage and Si diffuses into the semiconductor to form a leakage path and reduce the breakdown voltage of the AlGaN/GaN HEMTs. Moreover, N enrichment at the edge of the ohmic region of the LPCVD-SiNx device indicates that the device is more favorable for TiN formation, thus reducing the ohmic contact resistance, which is beneficial to improving the PAE of the device. Through the CW load-pull test with drain voltage VDS = 20V, LPCVD-SiNx devices obtain a high PAE of 66.35%, which is about 6% higher than MOCVD-SiNx devices. This excellent result indicates that the prospect of LPCVD-SiNx passivation devices used in 5G small terminals will be attractive. [ABSTRACT FROM AUTHOR]