Mobility & Scattering
In order for there to current, the electrons and holes must move. The ability
for them to move around in a material and transport charge is called mobility,
there's hole mobility and electron mobility. These are not necessarily
constants inherent to electrons and holes.
Several factors affect the mobility of a carrier. The most significant
is scattering, the motion-impending collisions within the crystal. These
collisions can be an electron bumping into another electron, or a hole
or ionized impurities. Scattering may increase/decrease due to temperature
and/or the addition of acceptors or donors. The higher the temperature
the more excited the carriers are, so the more scattering there will be.
The same occurs with dopants, because not only will there be more carriers
to bump into and scatter, dopants will generate ionized impurities which
also carry a charge. When the doping concentration is low, the mobilities
are pretty much independent of the doping concentration. The mobility of
the carriers begins to monotonically decrease as the concentration of the