Abstract: In the field of modern power electronics and RF applications, AlGaN/GaN High Electron Mobility Transistors (HEMTs) are highly valued for their outstanding electrical performance and high-power density. However, as demands for power density in these applications continue to grow, thermal management issues in AlGaN/GaN HEMT devices have become increasingly prominent, becoming a key factor limiting further performance enhancements, especially under high-power conditions where self-heating effects can lead to increased junction temperatures, thus affecting the reliability and lifespan of the devices. Therefore, developing new structures to optimize the thermal performance of AlGaN/GaN HEMTs is of significant importance for advancing the commercial application of this technology. Describes the self-heating effects of AlGaN/GaN HEMT devices with an AlN/SiN passivation layer, simulated using semiconductor device simulation tool(TCAD). The introduction of an additional AlN passivation layer on top of the device results in superior thermal performance compared to traditional devices with only a SiN passivation layer. The impact of different AlN thicknesses (from 0-10 µm) on the performance of GaN HEMTs is discussed, and the results show that devices with a 5 µm thick AlN layer exhibit better performance. This paper presents an AlN/SiN stacked passivation HEMT with a lattice temperature of 477 K, compared to 578 K for traditional SiN passivated HEMTs. It also compares the output characteristics, transfer characteristics, transient leakage current response curves, and gain-frequency characteristic curves of AlGaN/GaN HEMTs with an AlN/SiN passivation layer to those of traditional AlGaN/GaN HEMTs.