Metal-oxide semiconductor gas sensors are effective to air environment monitoring and indoor gas leakage detection, etc., because of the high sensitivity property. The principle of the sensor operation is the oxidation or the reductive reaction caused by gas molecules with the film surface heated to a high temperature.

The electrical resistance of the sensor changes by this reaction. It is possible to operate as a sensor of the impedance change type by adopting Interdigital Electrodes (IDEs) for the electrode that measures the electric characteristics of the Metal-oxide semiconductor film. The equivalent circuit of the sensor with IDEs depend on the heater voltage. When the general heater voltage is given (Drive at a high temperature), the sensor almost becomes pure resistance.

On the other hand, it operates as a parallel equivalent circuit of R-C when the voltage is set low (Drive at the vicinity of room temperature). In the case of high temperature driving mode, the sensor functions as a sensor of the change in resistance type by the oxidation and the reductive reaction of the old model. In the case of low temperature driving mode, the sensor functions as impedance changeable sensor by a conductivity change and a permittivity change of the film.

Those changes are caused by the physical adsorption of gas molecules. In this report, the principle of impedance changeable semiconductor gas sensor and the response characteristic of the sensor are described.