Work Function-Based Metal–Oxide–Semiconductor Hydrogen Sensor and Its Functionality: A Review

Hydrogen, a nonpolluting gas, is emerging as an ideal, suitable, and economical energy carrier. The current global non-carbon hydrogen production is 105.8 MW in 2020 and is expected to reach 218 MW in 2021. Hydrogen possesses low ignition energy of 0.017 mJ and reacts exothermically with air, posing severe safety challenges. Humanly undetectable gas needs accurate and sensitive sensors to prevent accidents. Amongst different hydrogen sensors currently developed, work function-based sensors are sensitive, selective, cost-effective, smaller in size, less susceptible to environmental change, and viable for mass production. This paper reviews semiconductor work function-based gas sensors, the structures and materials used in fabricating the structures, and the sensor performance. Metal–oxide–semiconductor (MOS)-based Schottky diode, MOS capacitor, and MOS field-effect transistors (FETs) are the three types of triple-layer work function-based sensors. The work function of the catalyst changes when the hydrogen molecule dissociates on its surface, ultimately causing an electrostatic potential shift, affecting the sensitivity. Nanomaterials and nanostructure add better sensitivity and response time to the sensor due to the high surface-to-volume ratio. Improved structures such as suspended gate FETs and hybrid suspended gate FETs may further improve the hydrogen adsorption into the catalyst due to the metal–oxide air gap without forming metal hydride.