4 - Effects

The sensing material in this gas sensor is metal oxide, most typically SnO₂. When a metal oxide crystal such as SnO₂ is heated at a certain high temperature in air, oxygen is adsorbed on the crystal surface with a negative charge. Then donor electrons in the crystal surface are transferred to the adsorbed oxygen, resulting in leaving positive charges in a space charge layer. Thus, surface potential is formed to serve as a potential barrier against electron flow. Inside the sensor, electric current flows through the conjunction parts (grain boundary) of SnO₂ micro crystals. At grain boundaries, adsorbed oxygen forms a potential barrier which prevents carriers from moving freely. The electrical resistance of the sensor is attributed to this potential barrier. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, so the barrier height in the grain boundary is reduced. The reduced barrier height decreases sensor resistance.

The principle or operation of this sensor is similar to the thick-film version. The adsorbed oxygen ions generate a depletion layer in the thin-film which results the increase of the resistance. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, thus decreasing the depletion layer and the resistance of the sensing material.