Electrical equivalent model for SnO2 bulk sensors

An electrical equivalent model based on electronic models of bulk ceramic SnO2 sensor for reducing gases is proposed. Assuming cylindrical and cubic shapes of particle, expressions for sensitivity have been derived as a function of the carrier concentration ni in the bulk interior of the particle, carrier concentration ns in the depletion region in the absence of sensing gas and carrier concentration nsp in the depletion region in presence of gas and also as a function of the ratio of depletion width d to the particle size D. It is also studied how the particle shape and the surfaces accessible to sensing gas affect the variation of sensitivity with particle size. A comparison of the theoretically evaluated sensitivity, with reported experimental data of SnO2, shows that a good agreement is obtained only when all surfaces of cylinder/cube have depletion region and are accessible to sensing gas. This indicates that the proposed model can qualitatively explain the dependence of sensitivity on particle size in SnO2 ceramic sensors. Assuming reasonable values of ni/ns and nsp/ns, the calculated depletion width is found to be independent of particle size and it comes out to be of the order of a Debye length.