2.4.1 Sensing mechanism of metal oxide semiconductor gas sensors

It is necessary to understand the sensing mechanism of the chemiresistive gas sensors for the subsequent chapters in this thesis. Since sensing mechanism of metal oxide semiconductor is mainly based on band theory, band theory can be applied to the gas sensor to explain the sensing mechanism. On interaction of the analytes (undetected) with the surface of nanostructures, these analytes react with attached oxygen ions on the surface of nanostructures; a change in the carrier concentrations of the material occurs. Due to the change in carrier concentrations of the material, the electrical resistivity of the materials changes. Decrease in resistivity (increase in conductivity) occurs for n-type metal oxide semiconductor on interaction of reducing gas [25]. So the sensing mechanism of oxide semiconductor is mainly based on the principle of modification in electrical properties (resistivity/conductivity) as a consequence of chemical reaction between gas molecules and the reactive oxygen ions on the surface of MOS nanostructure material. The sensing mechanism can be divided into three sections: (a) adsorption of oxygen at surface, (b) detection of gases by a reaction with adsorbed oxygen, and (c) change in resistance due to charge transfer at the surface.
