Stable Operation of AlGaN/GaN HEMTs at 400$^circ$C in air for 25 hours

In this letter, we report the operation of AlGaN/GaN HEMTs with Pd gates in air over a wide temperature range from 22$^circ$C to 500$^circ$C. The variation in the threshold voltage ($V_{th}$) is less than 1$%$ over the entire temperature range. Moreover, a safe biasing region where the transconductance peak ($g_m$) occurs over the entire temperature range was observed, enabling high-temperature analog circuit design. Furthermore, the operation of the devices over 25 hours was experimentally studied, demonstrating the stability of the DC characteristics and $V_{th}$ at 400$^circ$C. Finally, the degradation mechanisms of HEMTs at 500$^circ$C over 25 hours of operation are discussed, and are shown to be associated with the 2DEG sheet density and mobility decrease.