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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.

This paper is the first report of the high-temperature characteristics of a laterally vibrating piezoelectric lithium niobate (LiNbO$_{3}$) MEMS resonator array up to 500{\deg}C in air. After a high-temperature burn-in treatment, device quality factor (Q) is enhanced to 508 and the resonance shifts to a lower frequency and remains stable up to 500{\deg}C. During subsequent in situ high-temperature testing, the resonant frequencies of two coupled shear horizontal (SH0) modes in the array are 87.36 MHz and 87.21 MHz at 25{\deg}C and 84.56 MHz and 84.39 MHz at 500{\deg}C, correspondingly, representing a -3% shift in frequency over the temperature range. Upon cooling to room temperature, the resonant frequency returns to 87.36 MHz, demonstrating recoverability of device performance. The first- and second-order temperature coefficient of frequency (TCF) are found to be -95.27 ppm/{\deg}C and 57.5 ppb/{\deg}C$^{2}$ for resonant mode A, and -95.43 ppm/{\deg}C and 55.8 ppb/{\deg}C$^{2}$ for resonant mode B, respectively. The temperature-dependent quality factor (Q) and electromechanical coupling coefficient ($k_{t}^{2}$) are extracted and reported. Device Q decreases to 334 after high-temperature exposure, while $k_{t}^{2}$ increases to 12.40%. This work supports the use of piezoelectric LiNbO$_{3}$ as a material platform for harsh environment radio-frequency (RF) resonant sensors (e.g. temperature and infrared).

The current- and voltage-scaled sensitivities and signal-to-noise ratios (SNR) (with respect to thermal noise) of various octagonal AlGaN/GaN and InAlN/GaN Hall-effect sensors were examined in this work. The effect of metal contact lengths on sensitivity and sensor offset was evaluated. Calculations that take into account the shape of the device show that devices with point-like contacts have the highest current-scaled sensitivity (68.9 V/A/T), while devices with contacts of equal length to their non-contact sides have the highest voltage-scaled sensitivity (86.9 mV/V/T). The sensitivities of the two other devices follow the predicted trends closely. All the devices have offsets less than 20 $\mu$T at low supply current operation (< 300 $\mu$A) and most remain below 35 $\mu$T at higher supply current (up to 1.2 mA). The consistent low offsets across the devices imply that the choice of Hall-effect sensor geometry should mainly depend on whether the device is current-biased or voltage-biased and the frequency at which it will operate. This work demonstrates that GaN Hall-effect sensor performance can be improved by adjusting the geometry of the Hall-effect plate specific to its function (e.g., power electronics, navigation, automotive applications).