Re: Hot electrons in p-type and n-type silicon

chris.walker@xxxxxxxxxxx wrote:
Is there any evidence that hot electrons have different path lengths
in p and n type silicon ? The information that I have been given
suggests that inelastic scattering is dominated by e-h pair
production. Apparently the pair production scattering rates depend on
the electron energy with respect to the valence band maximum.
Therefore the path length should be different in p and n type, but so
far I have found no evidence for this....

First, do you really mean "hot electrons", i.e. majority carriers in n-type
and minority carriers in p-type material? Or do you mean "energetic majority
carriers"? In either case pair production (more commonly known as impact ionization)
cannot occur until the carrier has a kinetic energy exceeding the bandgap
energy. When this occurs, it doesn't matter if it is the dominant energy-loss
mechanism or not, because the multiplication of carriers overcomes any measurable
effects of the other mechanisms. Particularly if the energy source is a (fairly)
static electric field, impact ionization will lead to avalanche multiplication of
the carriers, as the newly-created electrons and holes are in turn accelerated and
cause ionizations. Also, high electric fields can only be maintained in regions that
are essentially depleted of carriers, so any n/p asymmetry would have to be due to
the ionized dopants, which would be a very much hihger-order interaction.

If you are thinking of bombarding a semiconductor with energetic electrons as in an
electron-beam induced current (EBIC) experiment, then again it is the kinetic energy
of the electron (i.e. how far above the conduction-band edge) that counts. In a
heavily p-type material one might imagine that the threshold energy for ionization
might shift a bit higher, as in the Moss-Burstein shift in optical absorption. Again,
this is likely to be a negligible effect.

As a general comment: Everything we normally teach about semiconductors applies in
or near thermal equilibrium. When we address far-from-equilibrium situations
(like energetic carriers) we have to be sure that the equilibrium concepts (like
n and p type) still make sense.

- Bill Frensley

.

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