The electrons in the conduction band and the holes in the valence band are free to move, carrying an electric charge. Therefore, they are called charge carriers or simply carriers. The number of charge carriers determines the magnitude of electric current. A detailed equation for the carrier density is omitted here, but it can be calculated as:
Figure 1-7 shows the energy states of undoped silicon. It has a very small amount of electrons in the conduction band and a very small amount of holes in the valence band. Having equal energy levels, these electrons and holes freely move around in a material so that its carrier density becomes uniform.
Figure 1-9 and Figure 1-10 show the carrier density curves of the n-type and p-type semiconductors, respectively.