Comparative study of hydrogen sensing characteristics of a Pd/GaN Schottky diode in air and N2 atmospheres

The hydrogen sensing and response characteristics of an interesting Pd/GaN Schottky diode under different concentration of hydrogen gases were systematically studied over a wide temperature range both in air and N2 atmospheres. During the hydrogen adsorption process, due to the presence of oxygen adsorbates on the Pd surface, the additional hydroxyl and water products were suggested to affect the H2 response of the sensors upon exposure to air. It revealed that the reaction of water formation appeared to be the rate-limiting factor and further inhibited the appearance of a dipolar layer at the Pd/GaN interface. Accordingly, the studied Pd/GaN Schottky diode exhibited higher hydrogen detection capability and larger Schottky barrier height modulation in N2 atmosphere in comparison with those in air. Based on the equilibrium adsorption analysis, the hydrogen adsorption heat ΔH° values were −18.24 and −24.74 kJ/mol in air and N2 atmospheres, respectively. Thus, the hydrogen detection capability was decreased with elevating the temperature. However, this phenomenon was more serious in air. In addition, according to the kinetic adsorption analysis, the activation energy Ea values were 34.86 and 4.53 kJ/mol in air and N2 atmospheres, respectively. This implied that the studied device could also perform more rapid detection of hydrogen in N2 atmosphere.