On the mass action law and the power law response in tin dioxide gas sensors

The electrical resistance of gas sensors, based on polycrystalline metal-oxide semiconductors, obeys a power-law response with the pressure of different gases (R ~ p^γ). The exponent γ can be derived resorting to the mass action law and its value depends on chemical reactions that take place at the surface of the grains. To explain the gas sensitivity, we revisit two conceptual models, regularly used in the literature: the ionosorption and the vacancy models. We show that they predict different values for the exponent γ. Also, the consequences of considering the bulk oxygen vacancies as deep levels are analyzed. Comparison of γ values obtained from both conceptual models with those found in experiments can indicate what mechanisms are possible to occur.