Nanotechnology-Enabled Sensors

6.3 Gas Sensing with Nanostructured Thin Films 311
Fig. 6.18 Different conduction mechanisms and changes upon exposure of a sensing
layer to O2 and CO. This survey shows geometries, electronic band pictures
and equivalent circuits. EC minimum of the conduction band, EV maximum of the
valence band, EF Fermi level, and λD Debye length. 32
The material can be completely or partly depleted which depends on the
thickness of the depleted layer that is formed on its surface after exposure
to a target gas and the Debye length λ D of the material. 32
Two types of sensitive layers (compact and porous) are presented in Fig.
6.19. 32 The contributions to the overall resistance corresponding to the
changes in the band bending at the material/grain surface and the potential
barriers that appear due to the metal/metal oxide contact are shown Fig.
6.19.
Although, the idea that only the resistance in the sensitive material is altered
when exposed to a target gas, is widely accepted; however, it is
viewed as an over-simplification. It should be remembered that the overall
resistance of the sensor depends not only on the gas sensing material properties
but also on other sensor parameters (transducer morphology, electrode,
etc.).