A MODEL FOR THE GAS-SENSING PROPERTIES OF TIN OXIDE THIN-FILMS WITH SURFACE CATALYSTS

The effect of two types of surface additives (Pd and Pt) on the response of reactively sputtered thin films of SnOx gas sensors has been studied, in mixtures of zero grade air and carbon monoxide. The experimental results obtained with surface additives show an abrupt conductance increase around 500 K when carbon monoxide in the ppm range is present, a behaviour which has not been observed with plain SnOx films. A semi-empirical model that explains this behaviour is presented, that is based on the well-established theory for the conductivity of ultrathin discontinuous metal films, i.e. activated charge carrier creation and tunnelling through potential barriers. The proposed model takes into account the dependence of film conductivity both on the thickness of the noble metal deposited on it and on the working temperature. The results of the theoretical analysis are in excellent qualitative agreement with the experimental ones.