  Because high electron mobility transistors (HEMTs) often exhibit significant
gate leakage during high-temperature operation, the choice of Schottky metal is
critical. Increased gate leakage and reduced ON/OFF ratio are unsuitable for
the design of high-temperature electronics and integrated circuits. This paper
presents high-temperature characteristics of depletion-mode molybdenum trioxide
(MoO${_3}$)-gated InAlN/GaN-on-silicon HEMTs in air. After a room temperature
oxidation of the Mo for 10 weeks, the leakage of the HEMT is reduced over 60
times compared to the as-deposited Mo. The use of MoO${_3}$ as the Schottky
gate material enables low gate leakage, resulting in a high ON/OFF current
ratio of 1.2 x 10${^8}$ at 25{\deg}C and 1.2 x 10${^5}$ at 300{\deg}C in air.
At 400{\deg}C, gate control of the InAlN/GaN two-dimensional electron gas
(2DEG) channel is lost and unrecoverable. Here, this permanent device failure
is attributed to volatilization of the MoO${_3}$ gate due to the presence of
water vapor in air. Passivation of the device with SiN enables operation up to
500{\deg}C, but also increases the leakage current. The suppression of gate
leakage via Mo oxidation and resulting high ON/OFF ratio paves the way for
viable high-temperature GaN-based electronics that can function beyond the
thermal limit of silicon once proper passivation is achieved.
