Analysis of thickness dependence of the sensitivity in thin film resistive gas sensors

A mathematical model for simulation of the steady state gas sensitivity (S=Gg/Ga) of a thin film resistive gas sensor is presented. The sensitive semiconductor film was taken to have formed from a finite number of independently operating polycrystalline layers. The target gas was assumed to affect the inner layers either by penetration through the grain boundaries or by direct interactions with all the layers at their exposed edges. The effect of micro-cracks and open pores were quantitatively accounted for as the providers of extra exposed edges. The simulation carried out predicted that in the micro-crack-free sensitive films, in general, the sensitivity reduces exponentially as the total thickness of the film increases. However, it was shown that this trend can profoundly be altered by the presence of the structural defects. It was also established that in micro-cracked or porous films the relationship between sensitivity and thickness is strongly dependent on the nature of the target gas; considerably different thickness dependences were predicted for the sensitivity to different target gases. The model was proved successful in explaining the experimental sensitivity versus thickness relationships reported for the gas sensors prepared by different fabrication techniques.