Information

Revised 6/03

GENERAL INFORMATION FOR TGS SENSORS

1. Operation Principle

The sensing material in TGS gas sensors 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

(Figure 1).

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 (Figures 2

and 3). The reduced barrier height decreases sensor

resistance.

The relationship between sensor resistance and the

concentration of deoxidizing gas can be expressed

by the following equation over a certain range of gas

concentration:

Rs = A[C]

−α

where: Rs = electrical resistance of the sensor

A = constant

[C] = gas concentration

α = slope of Rs curve

Fig. 1 - Model of inter-grain potential barrier

(in the absence of gases)

Fig. 2 - Scheme of the reaction between CO and adsorbed

oxygen on SnO2

Fig. 3 - Model of inter-grain potential barrier

(in the presence of gases)