Numerical computation of chemisorption isotherms for device modeling of semiconductor gas sensors

A computational method for numerical calculations of adsorption isotherms for both non-dissociative and dissociative chemisorption of gases on semiconductors is presented. The method enables calculating the equilibrium coverage of chemisorbed species and the chemisorption-induced potential barrier as a function of the ambient gas pressure, temperature, doping level, and the characteristic properties of the semiconductor/gas interaction. The computational method is applied for simulating the depletive chemisorption of oxygen on n-type SnO2. For both non-dissociative and dissociative chemisorption it is found that the chemisorption-induced potential barrier is proportional to the logarithm of the ambient oxygen pressure. This logarithmic relationship is important for device modeling of SnO2-based oxygen sensors since the sensor response, i.e. the change in the electrical conductivity, is related to the chemisorption-induced surface or intergranular potential barrier. The origin of this logarithmic relationship is attributed to the equilibration of the electrochemical potentials of chemisorbed oxygen adions and free oxygen molecules in the ambient gas phase.