Movement of electrons and holes

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.

Have a go! Use 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:

  • the electrons lie at the bottom of the conduction band; and 

  • the holes lie at the top of the valence band;

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.

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