A STUDY OF DUAL CONDUCTANCE RESPONSE TO CARBON-MONOXIDE OF CDS AND ALPHA-SNWO4 THIN-FILMS

An unusual dual conductance response was found in the we of exposure of n-type semiconductors CdS and alpha-SnWO4 in the form of thin films to carbon monoxide. Usually, CO behaves as a reducing gas increasing the conductance of n-type semiconductors. However, at temperatures below a border temperature T*, CO did behave as an oxidizing gas decreasing the conductance of CdS and alpha-SnWO4 thin films. T* for this dual conductance response to CO was found to be below 50 degrees C for CdS films. The enrichment of the surface with cadmium species had a decreasing effect on T* which was also decreasing with decreasing partial pressure of CO. For alpha-SnWO4 films, T* was near 250 degrees C depending on the partial pressure of CO. It was found that the dual response behaviour depends on temperature, CO concentration in air and tin content in the film.