Oxidation of thin film SnO2 layer was simulated. In particular, the evolution of depletion layer was investigated by solving Poisson-Boltzmann equation for SnO2 slab geometry grains. On this basis, the surface energy barrier dependence on layer thickness (30–500？nm) was obtained. The effect of the donor mobility (oxygen vacancies in the bulk) and degree of donor ionization on electric potential inside layer with different thicknesses was discussed. Furthermore, the dependence of per-square conductance on temperature (from 400？K to 700？K) has been computed. It was assumed that the bulk oxygen vacancies (donors) are singly or doubly ionized and mobile. The temperature variations in the carrier mobility were also taken into account. 1. Introduction Tin dioxide (SnO2) is a semiconductor oxide with a wide range of applications. The material is being used as a photocatalyst and a gas sensor and in optical devices [1–3]. In all these applications, the surface-related phenomena play a major role and become particularly important for nanomaterials, in which surface-to-volume ratio is much higher [4–6]. Precise numerical model of electrical properties of near-surface layer of SnO2 grains is fundamental for improving sensitivity and selectivity of SnO2 based gas sensors [6]. Shape and size of SnO2 nanograins are considered the most important factors for film conductance. It is caused by chemical reactions which take place on the surface of SnO2 grains. In particular, oxygen adsorption on the surface of SnO2 (ZnO and TiO2) decreases concentration of electrons in the near-surface layer. Depleted region is formed and conductivity of the layer is decreased. At the same time range of working temperatures of the film is raised from 420 to 720？K. Despite much experimental and theoretical research [7–19], the role played by adsorbed oxygen in surface and volume phenomena (surface coverage by oxygen ions, band bending, and distribution of mobile donors) occurring in nanolayers of SnO2 and the influence of these phenomena on conductivity have not yet been explained. Reversible changes in the conductivity of SnO2 layer have been reported, caused by a change in partial pressure of oxygen for sensor work temperature exceeding 500？K. The observed conductivity changes fulfil the exponential relation , where index assumes values from ？0.25 to ？0.5 or for temperatures above 1000？K [12]. Band bending caused by oxygen adsorption was reported by Mizsei and Lantto [13] and Semancik and Cox [14]. Also, X-ray photoelectron spectroscopy (XPS) was used to demonstrate band bending of
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