AlGaN/GaN high electron mobility transistors (HEMTs) are widely used in high frequency and power applications of the space and military industries due to their high RF power densities. When operated in full capacity, reliability of GaN HEMTs drop significantly due to device degradation, electron collapse phenomena, and concentrated heating effects. Although significant research has been done to investigate the effects of passivation, field-plates on the device degradation and the electron collapse separately, combined electrothermal analysis of the field-plates and the SiO2 passivation on GaN HEMTs has not been performed from the perspective of device reliability. For this purpose, electrothermal simulations of the field-plated and non-field-plated devices with different SiO2 passivation thicknesses are performed using Sentaurus TCAD to obtain the electrical field distribution and Joule heating caused temperature distribution in operating devices. Using these results, electrical and thermal effects of the field-plates on the devices with different SiO2 passivation thicknesses are analyzed to obtain the most effective and reliable operating conditions.