MOS gas sensors: What determines our choice?

Numerous researches have shown that a characteristic of solid-state gas sensors is the reversible interaction of the gas with the surface of Metal Oxide Semiconductor (MOS) materials. In addition to the conductivity change of gas-sensing material, the detection of this reaction can be performed by measuring the change of capacitance, work function, mass, optical characteristics or reaction energy released by the gas/solid interaction. Various materials, synthesized in the form of porous ceramics, and deposited in the form of thick or thin films, are used as active layers in such gas-sensing devices. However, in spite of so big variety of approaches to MOS gas sensor design the basic operation principles of all gas sensors above mentioned are similar for all the devices. As a rule, chemical processes, which detect the gas by means of selective chemical reaction with a reagent, mainly utilize MOS chemical detection principles. The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for MOS gas sensor applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity, type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity, and parameter's stability in sensing material choice as well.