  The electrical conductance of porous silicon fabricated with heavily doped
p-type silicon is very sensitive to NO$_2$. A concentration of 10 ppb can be
detected by monitoring the current injection at fixed voltage. However, 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
\micro\meter{} -- the injection decreases instead of increasing. We discuss the
effect in terms of an already proposed twofold action of NO$_2$, according to
which the free carrier density increases, and simultaneously the energy bands
are bent at the porous silicon surface.
