Conduction mechanism in undoped and antimony doped SnO2 based FSP gas sensors

The conduction mechanism in highly porous thick film SnO2 sensing layers, obtained by a direct deposition technique using FSP (flame spray pyrolysis) and the role of antimony (Sb) additives is investigated by simultaneous DC electrical resistance and work function changes measurements in various ambient atmospheres. The results have shown that the conduction mechanism changes according to the ambient atmosphere from a depletion layer controlled one to flat band situation and to an accumulation layer controlled one. In comparison to former studies performed on polycrystalline thick film layers, obtained from pre-processed powders, the measurements indicate that the grain size of the sensing material has a big influence on the conduction mechanism. In the case of undoped FSP-SnO2 sensors the studies showed that the grains are fully depleted in oxygen containing atmospheres. By increasing the number of free charge carriers in the bulk of the semiconductor, it was found that the addition of Sb changes the conduction mechanism dramatically. In contrast to the undoped material, there is a direct switch from a conduction mechanism controlled by the depletion layer to the degenerated semiconductor surface situation. Additionally, an initial upwards band bending is observed, even in the absence of atmospheric oxygen, which indicates a surface effect of Sb.