The effect of water vapor on the electrical properties and sensitivity of thin-film gas sensors based on tin dioxide

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 G0 on absolute humidity in the pure air is derived using a hypothesis of the presence of space-charge regions depleted of electrons between the SnO2 grains in a polycrystalline tin dioxide film. Due to dissociative chemisorption of water molecules, the energy-band bending at the SnO2 grain interfaces decreases and the oxygen-vacancy concentration in the grains increases, resuling in an increase in G0. An equation for the sensor response to hydrogen action is derived (the G1/G0, ratio, where G1 is the sensor conductance in a gas mixture containing molecular hydrogen). The expression describes the dependence of G1/G0 on the hydrogen concentration \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\rm n}_{{\rm H}_2 }$\end{document} in the interval 50–6·103 ppm, band bending at the SnO2 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.