Theory of gas sensitivity of nanostructured MOX layers: Charge carrier self-exhaustion approach

Starting from the charge carrier self-exhaustion theory (CCSE), we develop a logically closed theory of the conductivity of nano-structured semiconductor material (NSSM) valid in a large range of grain sizes a. The theory was formulated for the conditions, when the sensor interacts with target gas diluted in inert carrier, and then it was extended to the case of humid atmosphere. It is shown that high sensitivity to electronegative gases (O2, F2, etc.) is observed, when the grain size lies in a range of aSH < a < aCR, where aCR–is a critical size of a grain, below which charge carrier self-exhaustion regime takes place; aSH is a minimum grain size, below which the mechanism of superficial hopping conductivity predominates. This hopping mechanism excludes gas sensitivity of the material. Theorem of gas sensitivity of NSSM and the rules of the selection of nano-structures prospective for the fabrication of sensing MOX-layers are formulated. The physical concept, which enables taking into account the effects related to the presence of oxygen and humidity in the atmosphere and explaining the published experimental results, was developed.