Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Opposite effects of NO$_2$ 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 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.

1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
17
The categories of this paper are: q-fin
21
The categories of this paper are: adap-org
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
13
The categories of this paper are: quant-ph
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
3
The categories of this paper are: physics
37
The categories of this paper are: supr-con
28
The categories of this paper are: alg-geom
30
The categories of this paper are: q-alg
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex