A solid material consists of a large number of atoms held together by forces acting between their electrons.
When we bring atoms together, their individual electron energy levels combine to form energy bands.
An electron can only move from one atom to a neighbour if it can find a vacant position in the neighbour's orbits.
The energy level nearest to each atom, the lowest levels, are normally filled with electrons which are held very tighly by the nucleus.
The higher energy levels may be empty, but an electron on a lower orbit must obtain some extra energy as the 'entrance price'
if it wants to move to a higher orbit.
Electrons cannot move along bands which are complately full, and bands which are completely empty have no electrons to move. Hence
conduction can only take place in a band (from atom to atom at a given level) if the band is part-empty. In general, random thermal energy
inside a solid will give some electrons enough energy to jump up to the mostly empty conduction band. The 'free' electron can
then move around in response to an applied electric field (produced by an end-to-end voltage difference applied to the lump of material). These
free electrons movements let the material conduct electricity, i.e. a current appears when we apply a voltage. If you run the animation shown
above you will see that conduction can only take place by holes being transferred from atom to atom along the valence band.
When drawing diagrams that shows what happens to electrons inside a material or electronic device, it is simpler to ignore the individual atoms
and just draw some bands in which electrons must stay.
Using this picture of bands, we can imagine the current through a conductor as being a combination of electrons moving one way in the conduction
band and holes moving in the opposite direction in the valence band.
In reality, the situation is more complex than described here (isn't everything!?), but the basic idea of conduction by electrons in the conduction band
and holes in the valence band is good enough to understand 99% of electronics, so we don't need to say any more!
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Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)