I. Introduction
Various chemical sensors have been fabricated for monitoring chemical gases and ions [1]–[3]. Among those, sensors related to hydrogen detection have attracted considerable attention. This is mainly due to the expected replacement of hydrogen energy to fossil oil. Hydrogen is contained in nature and considered as a clean energy source [4]. However, hydrogen gas is flammable and explosive. The ignition of high-concentration hydrogen gas can possibly lead to a great explosion. Thus, the development of high sensitivity and fast response sensors is an important issue. With the great advance of semiconductor technology, considerable amount of solid-state electron devices such as high-electron mobility transistors have been fabricated [5]. By combining these electron devices with catalytic metals (e.g., Pd and Pt), the electrical properties can be changed by hydrogen-containing condition. These changes of the device characteristics are caused by the polarized hydrogen atoms at the metal–semiconductor interface [6]. Lundström et al. [7] first fabricated a silicon-based metal–oxide–semiconductor hydrogen sensing device. Since then, silicon-based hydrogen sensors were widely fabricated and comprehensively investigated [8]–[10]. Besides, III–V compound semiconductors, including InP, GaAs, and GaN for hydrogen sensors fabrication, also have received many attention. These materials have higher electron mobility and larger bandgap than silicon ones [11]–[20]. Furthermore, the III–nitride materials, e.g., AlGaN/GaN, are excellent candidates due to their inherent characteristics such as wide bandgap, resistance to chemical corrosion, high-temperature durability, and high-density 2-D electron gas (2-DEG). These properties give the promise for high-power and high-temperature microwave applications. The AlGaN/GaN-based structures exhibit better thermal durability because the bandgap of AlGaN can be modulated by adjusting the Al fraction. Wide bandgap of semiconductors can also reduce the thermal generation current and be employed in harsh environments. In addition, GaN-based materials exhibit less Fermi-level pinning effect than the others [18]. The pinning effect would limit the Schottky barrier height variation within a small range. Moreover, the spontaneous and piezoelectric polarizations can induce a high carrier density of 2-DEG in AlGaN/GaN heterostructure [19]. A high-density 2-DEG can make the sensors more sensitive to the surface condition, thus improving the hydrogen sensing capability [20]. It is speculated that the exposure to hydrogen can increase the sheet carrier density of 2-DEG, thus increasing the corresponding polarization-induced charge on the AlGaN surface [17]. The increase in polarization-induced charge might assist the formation of hydrogen dipole.