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Try this out...
We know that wavy things like electrons can sometimes appear localized, and
sometimes appear unlocalized.
When we speak of "charge carrier" we have in mind some localized entity that
appears to be moving or propagating.
If the entity does not possess some
degree of localization then I do not know how to picture "it" as being a
"charge carrier."
For example, I do not know how to describe the Hall
Effect without some localized charged entities that respond to the magnetic
field.
In some materials, experiments like the Hall Effect seem to indicate a
negative charge carrier. In other materials the same experiment seems to
indicate a positive charge carrier. I can picture this in my mind by
imagining that the charge carrier is the one that seems to have some amount
of localization to it. If the electron wave density in the conduction band
is giving rise to regions of localized negative density, then conduction in
this material will appear to negative charge carriers. On the other hand,
if the electron wave density in the conduction band is giving rise to
regions of localized positive density then that material will appear to have
positive charge carriers.
I introduce holes with a couple of (transparent) bottles of shampoo. Take
one that is mostly empty and turn is over - watch the shampoo flow down to
the bottom. Then take a nearly full bottle and turn it over - watch the
bubble rise.
Is something moving up in the second case? Well, not really - shampoo is
flowing down in both cases, but it seems like a completely different
phenomenon. And it is certainly easier to treat the motion of the bubble
(which moves smoothly) rather than the actual shampoo (which has many parts
each moving a short way and then stopping).