ELECTRICAL-PROPERTIES AND IMPROVEMENT OF THE GAS SENSITIVITY IN MULTIPLE-DOPED SNO2

The influence of temperature treatment as well as of multiple doping on the gas sensing properties of doped SnO2 was investigated by observing the morphological, electrical and gas sensing properties. Firing the different single-doped sensor elements at 600 degrees C, a linear dependence of the gas sensitivity on the resistivity of the sensor elements was found. The observed shift to higher gas sensitivities in comparison to sensors fired at 900 degrees C is caused by smaller crystallite sizes. This was proved by the determination of the specific surface area of the powders. However, the incomplete incorporation of the dopants into the bulk, in this case, may cause problems with long-term stability. Using multiple doping with Nb (resulting in finer powders) and Mn (high electron compensation), the synthesis of high electron compensated powders with small crystallite sizes is possible. The improvement of the gas sensitivity in comparison to single-doped powders was observed for a special range of doping concentrations. This improvement can be explained considering the relationship between an estimated electron concentration, crystallite size, thickness of the electron depletion layer and total number of electrons per crystallite.