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 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(H2) 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.