Silicon is a kind of semiconductor material whose number of free electrons is less than conductor but more than that of insulator. For having this unique characteristics, silicon has a vast application in the field of electronics. There are two kinds of energy band in silicon which are conduction band and valance band.
A series of energy levels having valance electrons forms the valance band in the solid. At absolute 0°K temperature the energy levels of the valance band is filled with electrons. This band contains maximum amount of energy when the electrons are in valance band, no current flows due to such electrons.
Conduction band is the higher energy level band which is of 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 valance band. This gap of energy is called forbidden energy gap. Actually this 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 very large gap for insulators. For semiconductors the gap is neither very large nor the bands get overlapped. Silicon has forbidden gap of 1.2 ev at 300°K temperature.
We know that in silicon crystal, covalent bond exists. Silicon is neutrally charged. When an electron breaks away from its covalent bond, a hole is created behind it. As temperature increases more, more electrons jump into conduction band and more holes are created in the valance bond.
Energy Band Diagram of Silicon
Energy band diagram of silicon shows the levels of energies of electrons in the material. There are two kinds of energy band, valance band and conduction band. Valance electrons occupy the valance band with highest energy level. Free electrons are in conduction band with minimum amount of energy. Valance and conduction bands are separated by the amount of energy known as the forbidden energy gap. This amount is nearly 1.2 ev at 300° K. In intrinsic silicon, the Fermi level lies in the middle of the donor atoms, it becomes n-type when Fermi level moves higher i.e. closer to conduction band. When intrinsic silicon is doped with acceptor atoms, it becomes p - type and Fermi level moves towards valance band.