The semiconductor sensor is using impedance device made of a kind of metal oxide film, its resistance changes with different gas content. The gas molecules react for reduction on the membrane surface to cause the change of the conductivity of sensor. In order to eliminate the gas molecules to the initial state, one oxidation reaction must occur. The heater inside the sensor can accelerate the oxidation process, which is why some low-end sensors are always unstable, the reason is that there is no heating or the heating low voltage is too low, thus causing the temperature to be too low and the reaction to be insufficient.