Lars-Gunnar Ekedahl (formerly Petersson) was born in 1947 in Kalmar, Sweden. He received a B.Sc. (1972) and a Ph.D. (1977) in physics from Linko ¨ping University. He became a Professor of surface chemical physics in Linko ¨ping (1993). He is also the program director of a new multidisciplinary graduate school, Forum Scientum, located at Linko ¨ping University. From 1981 his research has been focused on kinetics of surface reactions and catalysis, particularly with applications toward chemical sensing with MOS devices. Both modeling and experimental studies utilizing surface science oriented methods have been performed. At present Ekedahl heads a research group studying fundamental heterogeneous catalysis, from ultrahigh vacuum up to atmospheric pressures.Mats Eriksson was born in 1963 in Stockholm, Sweden. He received a M.Sc. in 1987 and a Ph.D in 1997 from Linko ¨ping University. He has studied fundamentals of hydrogen sensing with Pd-MOS structures in ultrahigh vacuum. He has studied adsorption processes and catalytic reactions on catalysts with different degrees of dispersion. He has also performed kinetic modeling of catalytic reactions.

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A simple electrostatic model of the adsorbate–adsorbate interaction of hydrogen atoms at a Pd–SiO2 interface is presented. The model predicts a hydrogen adsorption isotherm of the Temkin type. It is found that, in practice, an upper limit for the hydrogen response of a Pd-metal-oxide-semiconductor device exists. The value (in V) is equal to the difference of the initial heats of adsorption (in eV) of the interface and the Pd bulk, respectively. Furthermore, a corresponding maximum hydrogen concentration, at the interface, of 1×1018 m−2 is predicted. The predictions are in good agreement with previously observed experimental data.

The response of a Pd–SiO2–Si hydrogen sensor depends on the reaction kinetics of hydrogen on the Pd surface and on the hydrogen adsorption states at the Pd/SiO2 interface. In this work we show that besides the dominating hydrogen adsorption state located on the oxide side of the interface, a second state, resulting in opposite hydrogen polarization, exists. This state is possibly a reminiscence of the hydrogen adsorption state on a clean Pd surface. Taking both states into account, a simulation of the hydrogen response over more than ten decades in hydrogen pressures gives good agreement with published data.