Modelling of gas sensitivity for p-type semiconducting thin films

A model is presented for describing the conductivity dependencies of p-type thin film sensors from oxygen and CO pressures. The model basis on the kinetic equations, composed for the processes of dissociative ionization of O₂ and oxidation of CO on the surface, and taking into the account the charge balance between the adsorbed species and the hole accumulation layer. The model results are in agreement with the experimental data obtained on thin film sensors with two different p-type materials: Co₃O₄ and CoWO₄. Both the experimental and model data differ essentially from the data obtained previously for n type materials like SnO₂. In particular, it was found that the sensitivity to CO increases significantly with decreasing O₂ content in ambient environment. It is predicted that a similar effect (i.e. an increased sensitivity) can be obtained by engineering the intrinsic surface bending of the valence band.