Silicon is a semiconductor material whose number of free electrons is less than conductor but more than that of an insulator. For having this unique characteristic, silicon has a broad application in the field of electronics. There are two kinds of energy band in silicon which are conduction band and valence band. A series of energy levels having valence electrons forms the valance band in the solid. At absolute 0oK temperature the energy levels of the valence band are filled with electrons. This band contains maximum amount of energy when the electrons are in valence band, no current flows due to such electrons.
Conduction band is the higher energy level band which is the minimum amount of energy. This band is partially filled by the electrons which are known as the free electrons as they can move anywhere in the solid. These electrons are responsible for current flowing. There is a gap of energy between the conduction band and the valence band. This difference of energy is called forbidden energy gap. This gap determines the nature of a solid.
Whether a solid is metal, insulator or semiconductor in nature, the fact is determined by the amount of forbidden energy gap. Partially there is no gap for metals and large gap for insulators. For semiconductors, the gap is neither large nor the bands get overlapped. Silicon has forbidden gap of 1.2 eV at 300oK temperature.
We know that in a silicon crystal, the covalent bonds exist. Silicon is electrically neutral. When an electron breaks away from its covalent bond, a hole is created behind it. As temperature increases, more and more electrons jump into the conduction band, and more holes are created in the valence band.
Energy Band Diagram of Silicon
Energy band diagram of a silicon shows the levels of energies of electrons in the material. In intrinsic silicon, the Fermi level lies in the middle of the gap. When the intrinsic silicon is doped with donor atoms, it becomes n-type and then Fermi level moves higher i.e. closer to the conduction band. When intrinsic silicon is doped with acceptor atoms, it becomes p-type and Fermi level moves towards valance band.
Energy Bands Diagram of Intrinsic Silicon
Energy Bands Diagram of Extrinsic Silicon