|When an electron is promoted from the
valence, or any other band, to the conduction band, a positive hole is
left behind. Holes are also mobile. An electron within the valence
band may fill the hole, leaving another hole in its place. In
this way a hole appears to move.
In the presence of an electric field electrons move in one
direction and holes appear to move in the opposite direction. Since
both electrons and holes carry charge, they both contribute to the
conductivity of the material.
the simulation to illustrate the appearance of the movement of a hole
under an applied electric field as it is filled by electrons.
When electrons and holes redistribute themselves such that:
further excitation can occur giving a cascade of electron-hole
pairs. Under normal circumstances the extra excited electrons in the
conduction band might be expected to de-excite and return to the
valence band. However, in the presence of an electric field the holes
migrate up the field and the electron migrate down the field. This
allows collection of the electron-hole pairs.