| Summary: | We investigated the sensing characteristics of NO<sub>2</sub> gas sensors based on Pd-AlGaN/GaN high electron mobility transistors (HEMTs) at high temperatures. In this paper, we demonstrated the optimization of the sensing performance by the gate bias, which exhibited the advantage of the FET-type sensors compared to the diode-type ones. When the sensor was biased near the threshold voltage, the electron density in the channel showed a relatively larger change with a response to the gas exposure and demonstrated a significant improvement in the sensitivity. At 300 °C under 100 ppm concentration, the sensor’s sensitivities were 26.7% and 91.6%, while the response times were 32 and 9 s at <i>V</i><sub>G</sub> = 0 V and <i>V</i><sub>G</sub> = −1 V, respectively. The sensor demonstrated the stable repeatability regardless of the gate voltage at a high temperature.
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