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.
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