ELECTRON THEORY OF THIN-FILM GAS SENSORS

The simple charge-transfer model usually applied to thin-film gas sensors is critically discussed and the fundamental difficulties are elucidated from a quantum-chemical point of view. In order to overcome the inconsistency of the charge-transfer model, a microscopic model of a thin-film gas sensor is developed in the framework of the electron theory of chemisorption. On the basis of Volkenstein's theory, two forms of chemisorption are introduced: the neutral weak-chemisorbed form and the charged strong-chemisorbed form. For the case of acceptor-like chemisorption and n-type metal-oxide semiconductors, the pressure dependence of the thin-film conductivity is derived, solving self-consistently the one-dimensional Poisson equation. Depending on the geometric and electronic structure of the thin-film gas sensor, relationships are derived in a consistent manner between the catalytic surface properties and the electronic bulk properties. It turns out that different types of adsorption isotherms can arise, depending on the bulk doping level. An interesting feature of the catalytic behaviour of the proposed microscopic model is the saturation effect of charged strong-chemisorbed species, which already arises at very low partial pressure (often known as the Weisz limitation in the sensor literature). In contrast to the charged strong-chemisorbed species, the neutral weak-chemisorbed species show a typical Langmuir behaviour. The main result is the power-law behaviour of the thin-film conductivity over a wide pressure region, sigma(p) approximately p(-m). Whereas in the sensor literature the different power laws are explained by applying mass-action laws, it is shown that the power, m, is determined by different, fundamental parameters: xi = D/L(D) (L(D) = Debye length), film thickness D, temperature and the surface-state energy. Hence it cannot be described by the stoichiometric coefficient alone. Furthermore, the theoretical values of the power vary between 0 and 1, in agreement with various experimental results. A modified Helmholtz equation is proposed, which can connect the change of the work function to both the change of the surface potential due to the charged strong-chemisorbed species and the change of the electron affinity due to the dipole moment of the neutral, weak-chemisorbed species. The effect of an external field on chemisorption is investigated for the case of a simplified one-dimensional 'suspended'-gate MESFET (metal-semiconductor FET).