The Electroadsorptive Effect and its Application in Semiconductor Gas Sensors
Prof. Dr. Theodor Doll (Institut für Mikrotechnik Mainz GmbH)

One of the major problems in chemical sensing with thin film semiconductor sensors is related to the reversibility of chemical reactions. Reversibility means that, in terms of gas sensors, an adsorbed layer should leave the surface rather fast after the end of a gas exposure bringing the sensor signal back to its base line. This is influenced by transport processes (charge carriers and gas diffusion), but mainly depending on reaction enthalpies (binding energies) not exceeding several kT which is the thermal energy of the bound gas molecule. On the other hand, for significantly high responses of the sensor a large amount of adsorbed species is desired under gas exposure, which requires higher binding energies. Additional problems arise from very strong binding sites that are present on real surfaces causing stable, preadsorbed or memory layers.
Up to now, changing the sensor operating temperature has been the common way to tradeoff with these problems. However, from a solid state physics view, changing the temperature is equivalent to changing the statistic equilibriums of transport and adsorption processes as mentioned above. There is little difference towards chemistry except adding Fermi statistics to Boltzman where necessary: The Fermi level governs the surface states, which their selves lead to sensor signals according to alterations of the relative position of these states towards the Fermi level. Conversely, if one artificially changed these relative positions this would result in a change of the probability that adsorption at a sensor surface can occur. This means that sensing itself gets under control.
The most powerful tool to do this is using the electrical field effect in semiconductor surfaces, being just the physical basis of most modern electronics including the silicon part of the gas sensitive MOSFET mentioned above.
An electrical field perpendicular to a semiconductor surface induces some alteration of the band bending and will move the whole scheme of surface bands and surface states in their relative positions to the Fermi level, and thus changes the adsorption statistics of the surface, as given in the Figure. This is the basic idea of the electroadsorptive effect as firstly described by Wolkenstein . Based on his work we developed semiconductor simulation models and designed and fabricated thin film gas sensors that have proven this concept. In our contribution we will show own results and give an overview to related work of other groups as well discuss problems and applications of the electro adsorptive effect in micro gas sensors.

[1]Th. Wolkenstein, The Electron Theory of Catalysis on Semiconductors, McMillan, N.Y. 1963