Selectivity enhancement of gas sensors using non-equilibrium polarisation effects in metal oxide films

The gas sensing behaviour of granular tungsten oxide films under the influence of a polarizing electrical field is investigated. The Ohmic resistance of the film in the direction of applied voltage is altered in a reversible way due to ionic transport processes in the film. Several processes with a wide range of time constants from ∼100 ms to 100 s occur. A model is proposed to describe the movement of bulk donors and related changes of the surface charge. Within the model sensor instabilities are described which can be prevented by a pulse technique for readout. As an benefit a single sensor element used in different polarization states can discern several gases.