The discovery in 1953 that adsorption of a gas onto the surface of a metal oxide semiconductor produced a large change in its electrical resistance signalled the advent of mixed metal oxide semiconductor sensor (MMOS) technology. The effect is commercially exploited for only a few oxides due to the requirement for a unique combination of resistivity, magnitude of resistance change in gas (sensitivity) and humidity effects.
Amongst the oxides which are used as MMOS sensors are
ZnO2, TiO2, Cr2TiO3, WO3 and SnO2.