Moving Charges (Electrons& Atoms)

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.

(Image :Moving Charges) 

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.                         

(Image: Energy Band Diagram)