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.
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