GaN-based metal/insulator/semiconductor-type schottky hydrogen sensors

Hydrogen (H₂) gas has many interesting and useful properties and has been widely used as hydrogenating and reducing agents in numerous industrial branches, such as petroleum, chemical, metallurgical, and foodstuff industries. It is also used in power stations as a coolant in electric generators. Nowadays, environmental pollution has become a major issue. Hydrogen, as one of potentially clean energy sources, has attracted intensive attention. Hydrogen gas is expected to be one of the principal energy sources in the future and can be used in future power devices, solid oxide fuel cells, H2 engine cars, and so on [1-6]. On the other hand, hydrogen gas is hazardous, is highly inflammable, has low ignition energy, and is explosive at room temperature when its concentration in air exceeds about 4%. However, hydrogen gas is odorless and colorless, so it is difficult to be detected by human sensing organs. Furthermore, because hydrogen’s small molecular size promotes leaks and diffusion, leaking hydrogen gas warms, and may spontaneously ignite due to its negative Joule-Thomson coefficient at room temperature. Therefore, to ensure safety it is necessary to detect the leakage and monitor its existence and concentration in the surrounding atmosphere using specific detectors wherever hydrogen is produced, transported, stored, or used. Over time, much effort has been made to exploit various reliable, durable, accurate, sensitive, fast to respond, easy to operate, and inexpensive hydrogen sensors. Based on the variation of various physical-chemical properties of materials on exposure to hydrogen-containing ambience, different types of hydrogen sensors have been developed [7].