Opposite effects of NO2 on electrical injection in porous silicon gas sensors

The electrical conductance of porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO2. We show that the sign of the injection variations depends on the porous layer thickness. If the thickness is sufficiently low-of the order of few mum-the injection decreases instead of increasing. We discuss the effect in terms of an already proposed twofold action of NO2, according to which the free carrier density increases, and simultaneously the energy bands are bent at the porous silicon surface. (C) 2004 American Institute of Physics.