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We report an AlGaN/GaN HEMT gas sensor designed to enable NO, NO2, and NH3 detection from 100 °C–400 °C over a large concentration range. Device modeling is performed to optimize several HEMT device parameters for sensing, and the experimental results show that the optimized sensor has improved performance compared with the previously reported HEMT sensors. The device shows significant no sensitivity for the first time in an HEMT device, with sensitivity up to 7% at 400 °C. In addition, high sensitivities of up to 17% are reported for NO2, and NH3 is detected at concentrations as low as 150 ppb. [ABSTRACT FROM PUBLISHER]