Sensors - Hierarchical Nanostructures for Energy Devices - page 251

d n 3 r 4 n g | 4

Figure 11.2 Electrical sensing mechanism of an n-type metal oxide nanostructure
gas sensor: (a) surface adsorption of oxygen, (b) microstructure of the
sensing layer, (c) measurement of output resistance, and band model
showing (d) initial state and (e) effect of reducing gases on space-
charge layer (L air ) and Schottky barrier (eV surface ) (adapted from ref. 15
with permission).

.

embedded heater. On the other hand, oxidizing gas removes electrons on the
surface of p-type materials leaving holes which play the role of charge
carrier, producing higher conductance. The electrical resistance/conductance
is also dependent on the interparticle barrier, known as the 'Schottky barrier'
as shown in Figure 11.2(d) and (e). As more electrons are trapped by
ionosorbed oxygen, a wider space-charge layer and higher Schottky barrier
are induced. The space-charge layer becomes thinner during the gas reactions
above, so that a lower Schottky barrier along the percolation path exists.
In gas sensors, there are two main quantities to evaluate sensor per-
formance. One is the gas response that is defined as the ratio of resistance
when the sensor is exposed to ambient air (R a ) to that when it is subjected to
the target gas (R g ). It is very similar to the sensitivity in that a higher gas
response is expected at higher gas concentration unless the surface is not
saturated producing stiffer behavior in a gas response versus concentration
curve. The other performance is the response/recovery time, which was de-
fined as the time required for reaching 90% of the final equilibrium state