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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.

The magnetic sensitivity of Hall-effect sensors made of InAlN/GaN and AlGaN/GaN heterostructures was measured between room temperature and 576{\deg}C. Both devices showed decreasing voltage-scaled magnetic sensitivity at high temperature, declining from 53 to 8.3 mV/V/T for the InAlN/GaN sample and from 89 to 8.5 mV/V/T for the AlGaN/GaN sample, corresponding to the decreasing electron mobility due to scattering effects at elevated temperatures. Alternatively, current-scaled sensitivities remained stable over the temperature range, only varying by 13.1% from the mean of 26.3 V/A/T and 10.5% from the mean of 60.2 V/A/T for the InAlN/GaN and AlGaN/GaN samples respectively. This is due to the minimal temperature dependence of the electron sheet density on the 2-dimensional electron gas (2DEG). Both devices showed consistency in their voltage- and current-scaled sensitivity over multiple temperature cycles as well as nearly full recovery when returned to room temperature after thermal cycling. Additionally, an AlGaN/GaN sample held at 576{\deg}C for 12 hours also showed nearly full recovery at room temperature, further suggesting that GaN-based Hall-effect sensors are a good candidate for use in high temperature applications.