1.04 Energy band diagrams
Energy band diagrams show the energy levels of the electrons in the material. We are only interested in two of the bands, the conduction band and the valence band. The valence band is occupied by the electrons with the highest energy level of those which are still attached to their parent atoms, these are the outer most (or valence) electrons. The conduction band is occupied by electrons which are free from their parent atoms. These electrons are free to move through the material. (When a voltage is applied these electrons will drift to produce an electrical current.) In semiconductors there is an gap between the valence and conduction bands. This energy gap reflects the amount of energy that would be needed to remove an electron from it’s parent atom (ie to transfer it from the valence to the conduction band).
Energy band diagram for silicon
The number of valence electrons in the pure silicon is enough to completely fill the valence band (so no movement of electrons can occur in the valence band). There are no free electrons, therefore the conduction band is completely empty.