An interesting hydrogen sensor based on a Pd/HfO2/GaOx/GaN metal-oxide-semiconductor (MOS) structure is fabricated and demonstrated. The HfO2 and GaOx layers are prepared using a sputtering approach and hydrogen peroxide (H2O2) treatment. The hydrogen sensing characteristics of the studied device are comprehensively studied. Experimentally, good hydrogen sensing characteristics, including a high sensing response of 8.47 x 10(5), a low detection level of 5 ppm H-2/air, and reversible, short response and recovery times upon exposure to different hydrogen concentrations and temperatures are obtained. The influence of humidity on hydrogen sensing performance is also studied. The exothermic action of the hydrogen adsorption process leads to a decreased hydrogen sensing response at higher temperatures. Consequently, the studied Pd/HfO2/GaOx/GaN MOS diode is promising for high-performance hydrogen sensing applications and integration with other GaN-based high-speed devices on a chip. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.