Semiconductor gas sensors are based around a metal oxide, normally SnO2. When a metal oxide crystal is heated to a certain high temperature, specific elemental molecules are adsorbed by the crystal surface with a negative charge. The sensor can be sensitised to different gases by the choice of operating temperature, microstructural modification and the use of dopants and catalysts.

The donor electrons from the surface of the crystal are transferred to the adsorbed oxygen, leaving positive charges in a space charge layer and creating a potential barrier against electron flow. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, decreasing the barrier height in the grain boundary.

New nanostructure materials are boosting the performance and sensitivity of semiconductor gas sensors due to their much higher surface-to-bulk ratio.