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 n Н 2 are derived for all types of sensors. The relations describing the dependences of the SnO 2 -sensor conductivity and response on the absolute humidity of a gas mixture are given. It is shown that the relaxation time τ rel of the response of SnO 2 -and MOS-structure sensors is determined by the relaxation time τ а of hydrogen atom adsorption on the SnO 2 and SiO 2 surfaces, respectively. For the MOS-diodes, τ rel = τ а at n Н 2 <3⋅10 3 ppm and τ rel = τ d at n Н 2 ≥ 7.5⋅10 3 ppm, where τ d is the relaxation time of hydrogen atom diffusion through an SiO 2 layer.
The results of theoretical and experimental studies into the effect of water vapor on the electrical conductance of a gas sensor and the sensor response to hydrogen action are discussed. A relation describing the dependence of electrical conductance G 0 on absolute humidity in the pure air is derived using a hypothesis of the presence of space-charge regions depleted of electrons between the SnO 2 grains in a polycrystalline tin dioxide film. Due to dissociative chemisorption of water molecules, the energy-band bending at the SnO 2 grain interfaces decreases and the oxygen-vacancy concentration in the grains increases, resuling in an increase in G 0 . An equation for the sensor response to hydrogen action is derived (the G 1 /G 0 , ratio, where G 1 is the sensor conductance in a gas mixture containing molecular hydrogen). The expression describes the dependence of G 1 /G 0 on the hydrogen concentration n H 2 in the interval 50-6·10 3 ppm, band bending at the SnO 2 grain interface, and sensor temperature. The dependences of the sensor conductance, highest possible conductance, and energy-band bending on temperature and absolute humidity resulting from processing of the experimental data are in good agreement with the theoretical predictions.
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