Nowadays, high power and high frequency devices have various reliability issues due to which their performance is limited. The main cause is self-heating in gate drain region and E-field crowding near gate edge of AlGaN/GaN HEMT. In this work, our aim is to study and analyze the self-heating effects of field plate AlGaN/GaN-based high-electron mobility transistors (HEMTs) grown on sapphire substrates employing a dual AlN/SiN passivation using electro-thermal simulations. It is observed that the temperature of the proposed device is reduced from 578 to 439 K by incorporating the passivation layer with field plate in AlGaN/GaN which is \(\sim \)  24% reduction in device temperature. The proposed device with dual passivation layer has \(\sim \)  41% improvement in drain current, \(\sim \)  14% in transconductance, and \(\sim \)  87% reduction in E-field at the edge of device.