In a crystal, the discrete energy levels of the individual atom broaden into energy bands.
Each quantum state of the individual atom gives rise to a certain energy band. The
bonding combinations of states become the valence bands (VB) of the crystal, and
the anti-bonding combinations of these states become the conduction band (CB). The
energy difference between VB and CB is called energy gap. If the valence bands are
partly filled, this material is p-type, if the conductive bands are partly filled, this
material is n-type. Here Fermi level is used to label the occupation conditions of
electrons in the semiconductor, it is the energy level to which electrons occupy. Fermi
level (EF P ) on p-type is near the valence band and EF N on the n-type is near the
conductive band