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The conduction mechanism in highly porous thick film SnO 2 sensing layers, obtained by a direct deposition technique using FSP (Flame Spray Pyrolysis), and the influence of different antimony (Sb) doping levels on the electrical properties of the host material is investigated by simultaneous DC electrical resistance and work function changes measurements in various ambient atmospheres. The situation in the case of the undoped FSP-sensors is very similar to the results obtained on polycrystalline thick film layers: we found a seamless transfer of the conduction mechanism from a depletion layer controlled one to the flat band situation and to an accumulation layer controlled one. The switch between these two models directly occurs in the absence of ambient oxygen, indicating that the depletion region is only determined by its adsorption. The degeneration of the semiconductor, in the case of the accumulation layer controlled conduction mechanism, is not observed. The addition of Sb influences the conduction mechanism dramatically due to its effect on the bulk concentration of free charge carriers. In contrast to the undoped FSP-SnO 2 sensor there is a direct switch from a conduction mechanism controlled by the depletion layer to the degenerate semiconductor situation that indicates a deep crossing of the Fermi level into the conduction band; moreover an initial upward band bending is observed.

A model for sensing with semiconducting metal oxide (SMOX)-based gas sensors was developed which takes the effect of the shape of the grains in the sensing layers into account. Its validity is limited to materials in which the grains of the SMOX sensing layer are large enough to have an undepleted bulk region (large grains). This means that in all experimental conditions, the SMOX properties ensure that the influence of surface phenomena is not extended to the whole grain. The model takes the surface chemistry and its impact on the electrical properties of the sensing material into consideration. In this way, it relates the sensor signal--defined as the relative change of the sensor's conductance--directly to the concentration of the target gas and also exhibits meaningful chemical parameters, such as the type of reactive oxygen species, the reaction constants, and the concentration of adsorption sites. The validity of the model is confirmed experimentally by applying it to data gathered by measuring homemade sensors in relevant conditions.

The variation of the calcination temperature in the fabrication procedure of sol-gel SnO 2 thick film gas sensors leads to very different sensing characteristics [1]. In the present studies, the conduction mechanism and electrical characteristics of the surface of undoped SnO 2 based sensing materials -calcined at different temperatures -are investigated by performing DC electrical resistance and work function changes measurements. The results show for the first time the existence of an intrinsic surface band bending for undoped SnO 2 (calcination temperature 450°C). Because an intrinsic surface band bending has a big impact on the conduction mechanism and the concentration of surface species, which have an influence on the electrical resistance, the sensing performance is dramatically changed. The effect is similar to 1 at.% Ni surface loading, which showed a huge intrinsic surface band bending and an enhanced sensing performance in comparison to the undoped base material [2].

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