Extension of receptor function theory to include two types of adsorbed oxygen for oxide semiconductor gas sensors

Receptor function of oxide semiconductor gas sensors to oxygen and an oxidizing or reducing gas was extended successfully to include two types of adsorbed oxygen, O ^- and O ^2-. The receptor function relying on O ^2- ions is featured by linear dependence of resistance on PO ₂1/4 (PO ₂, partial pressure of oxygen), while that relying O ^- shows linear dependence on PO ₂1/2. As a result, the resistance can be brought to a high value at small PO ₂ when O ^2- ions are formed. The drastic increase of resistance in air under desiccated condition can be accounted for as a result of the formation of O ^2- ions which is blocked by water vapor in humid conditions. On the response to an oxidizing gas (NO ₂) in air, O ^2- ions exert an adverse effect; those increase the gross resistance of the device in air, thus reducing the conventionally defined response to the gas. Those influence on the response to a reducing gas (H ₂) in a particular way. Owing to a kinetic reason, O ^2- ions are almost distinguished at steady state under exposure to the gas, rendering only O ^- ions to be responsible for the response to the gas, and thus contributing to amplify the conventionally defined response.