On the apparently anomalous response of porous silicon to nitrogen dioxide

Recent publications reported a large increase of conductivity in mesoporous silicon exposed to traces of nitrogen dioxide. The phenomenon has been studied from theoretical and experimental points of view. Even if a complete comprehension has not been reached yet, it seems clear that the simple increase in the number of mobile carriers inside the material cannot fully account for the experimental evidence; an increase of their mobility must provide a more robust contribution. The fresh observation of an opposite effect occurring in sufficiently thin mesoporous samples is then surprising. The present letter shows that such an apparently contradictory result can be explained in terms of the well-known theory of current injection in insulators. (C) 2004 American Institute of Physics.