Abstract: The roles of micro-metal–oxide (MO) interfaces inside a sensing metal formed by coevaporating Pd and SiO2 in metal–semiconductor–metal GaN sensors are investigated. The porous property of the Pd and SiO2 mixture together with the presence of micro-MO interfaces gives rise to a highly efficient dissociation of hydrogen molecules and hence an enhanced barrier height variation (ΔphivB) of a reverse-biased Schottky diode. The measured ΔphivB increases from 294 to 392 mV at a concentration coefficient of 25 mV/decade as the hydrogen concentration increases from 2.13 to 10100 ppm H2/N2. Therefore, when the sensor is subjected to 0.02 ppm H2/N2, ΔphivB as high as 245 mV is still expected. The sensor in a 2.13 ppm H2/N2 ambience has a sensing response of 8.7×104. Excellent dynamic responses are demonstrated by switching voltage polarity or continuously changing hydrogen concentration, showing that the proposed structure is a promising hydrogen sensor.