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 O2−. The receptor function relying on O2− ions is featured by linear dependence of resistance on PO21/4 (PO2PO2, partial pressure of oxygen), while that relying O− shows linear dependence on PO21/2. As a result, the resistance can be brought to a high value at small PO2PO2 when O2− ions are formed. The drastic increase of resistance in air under desiccated condition can be accounted for as a result of the formation of O2− ions which is blocked by water vapor in humid conditions. On the response to an oxidizing gas (NO2) in air, O2− 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 (H2) in a particular way. Owing to a kinetic reason, O2− 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.