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The detection of hydrogen in the Earth's deep underground poses a major challenge due to the lack of oxygen and continuous changes in environmental conditions. An innovative class of hydrogen gas sensors based on AlGaN/GaN High Electron Mobility Transistors (HEMTs) with Platinum (Pt) gates as a functionalization layer, has been developed and optimized for geo-sensing. The study investigated the sensor characteristics using two carrier gases to simulate underground conditions, namely air (with 20 % O 2) and N 2 (with 0 % O 2), across a range of temperatures from 50 °C to 300 °C and with hydrogen concentrations varying from 25 ppm to 400 ppm. The detection limit was found to be approximately 1 ppm of hydrogen in the atmospheric air. The gas sensor transduction is based on the modification of the conductivity of a 2-Dimensional Electron Gas (2DEG). In this study, the principle was investigated using two gases, O 2 and H 2 , with different electronegativities relative to platinum. The adsorption competition between H 2 and O 2 on platinum was evaluated, and this allowed the calculation of the ratio of thermodynamic adsorption constants between these two gases. • Investigated the Pt-AlGaN/GaN HEMT-based sensor for detecting underground H 2 gas using N 2 and air as carrier gases. • Key parameters for hydrogen sensing included the effects of temperature and hydrogen concentration. • Demonstrated that hydrogen has 2000 times greater affinity than O 2 with the Pt-AlGaN/GaN sensor functional layer. • Presented and comprehensively explained the adsorption kinetics of hydrogen with the Pt functional layer. [ABSTRACT FROM AUTHOR]