Thermal modelling of a WO3 ozone sensor response

Electrical sensitivity to oxidizing gases of sputtered tungsten trioxide (WO3) thin films are investigated by measuring the electrical conductivity versus ozone concentration in a range from 0 to 0.8 ppm at different operating temperatures. At 523 K (250 °C), the resistivity is divided by a factor close to 103 when the ozone concentration decreases from 0.8 ppm to 0. The charge transport mechanisms depend on the adsorption-desorption balance of the atmospheric gases (oxygen and ozone) and on the reactivity of the WO3 thin-layer surface. A first (analytical) model takes into account the adsorption effect to determine the electrical charge located at the surface of each grain. A second model of microcrystalline porous WO3 thin film derived from the theory of semiconductor devices is developed to compute the conductivity of the layer. The effects of the two mechanisms involved are analysed and a comparison between simulation and experiment is done.