A diffusion-reaction model of electrical conduction in tin oxide gas sensors

Abstract

The recent advent of integrated gas sensors and advances in pattern recognition techniques have heralded a renewed interest in the electrical characteristics of porous thick semiconductor layers. A linear diffusion-reaction model is considered here that may be used to characterise the transient and steady-state response of several metal-semiconductor-metal devices. This approach differs from that of chemical kinematics which is widely adopted and focuses upon the relationship between electrical conductance and the geometrical structure of the device. The theoretical transient response is derived for several MSM device configurations and compared with experimental data on SnO₂ gas sensors. There is reasonable agreement with the model predictions at low gas concentrations. Consequently, the electrical conductivity of thick porous tin oxide gas sensors is diffusion-limited as the reaction rate is fast compared with the diffusion rate. However, further work is needed to address nonlinear adsorption processes and to consider a diffusion-reaction process where the reaction rate is slow compared with physical diffusion.