SENSING MECHANISM IN TIN OXIDE-BASED THICK-FILM GAS SENSORS

A model has been developed for the sensing mechanism of metal oxide-ba sed thick-film gas sensors. The model explains the behaviour of the se nsor conductance as a function of the concentration of test gas and th e operating temperature of the sensor. Using the Schottky-barrier cond uction mechanism through grain boundaries, a relationship between the degree of surface coverage theta and the conductance G has been obtain ed. To relate the conductance with the concentration of the gas, the F reundlich adsorption isotherm for gases and vapours on a solid surface has been used. The isotherm relates the degree of surface coverage th eta with the partial pressure of the gas (concentration). By eliminati ng theta, an expression relating the variation of G with concentration has been obtained. To study the validity of the model, a thick-film P d-doped tin oxide gas sensor has been fabricated and tested with propa nol (C3H7OH). The variation in the conductance with changes in concent ration and temperature has been observed. The observed data show an ex cellent fit with the developed model. Using the experimental data, the constants of the theoretical equation have also been evaluated.