Conduction mechanism switch for SnO2 based sensors during operation in application relevant conditions; implications for modeling of sensing

In this paper we present experimental evidence for a conduction mechanism switch for gas sensors based on SnO2 porous, thick film sensing layers under exposure to CO in the presence of humidity. In dry air and CO concentration below 250 ppm the conduction is controlled by the surface depletion layers formed by the trapping of electrons from conduction band on adsorbed oxygen species. In the presence of humidity and at CO concentration exceeding 3 ppm the surface depletion layers are replaced by surface accumulation layers and the conduction mechanism changes accordingly. We also present a model that takes into account the influence of humidity at low CO concentrations.