Basic physics of semiconductor hydrogen sensors

The most probable physical models of hydrogen sensors based on thin stannic oxide films, MOS-structures, and tunnel MOS-diodes are discussed. The emphasis is on the mechanisms of formation of sensor response to hydrogen. The analytical equations describing the dependence of the response on the hydrogen concentration nH2 are derived for all types of sensors. The relations describing the dependences of the SnO2-sensor conductivity and response on the absolute humidity of a gas mixture are given. It is shown that the relaxation time τrel of the response of SnO2-and MOS-structure sensors is determined by the relaxation time τa of hydrogen atom adsorption on the SnO2 and SiO2 surfaces, respectively. For the MOS-diodes, τrel = τa at nH2 and τrel = τd at nH2≥7.5·103, where τd is the relaxation time of hydrogen atom diffusion through an SiO2 layer.