Pt/n-GaN and Pd/n-GaN Schottky diodes were characterized for their response to hydrogen as a function of measurement temperature. Even at modest temperatures (80-140 degreesC), an 80 mum diameter diode shows a large increase ( greater than or equal to 0.5 mA) in forward current upon introduction of similar to10% H-2 into a N-2 ambient. The change in current increases with measurement temperature and begins essentially immediately upon introduction of the hydrogen. Cycling the ambient from N-2 to 10% H-2 in N-2 and back to N-2 produces reproducible cycling of the forward current at fixed forward bias. The decrease in barrier height of Pt on GaN was 50 mV at 25 degreesC and 70 mV at 150 degreesC upon introduction of H-2 into the ambient, with lower values for Pd. At high temperature, the time response of the sensors appears to be controlled by hydrogen diffusion to the metal/GaN interface, while at low temperatures (<100&DEG;C), dissociation of the gas appears to be the rate-determining step. (C) 2003 Elsevier Science Ltd. All rights reserved.