Energy band or energy band gap in solids is defined as the energy gap between the top of the valence band and bottom of the conduction band. The unit of the energy band gap is normally electron-volt (ev). It is denoted by the symbol E_g.

According to Bohr’s atomic model, the electrons revolving around the nucleus are governed by a definite path and hence have a definite energy level so the electrons in an atom have well-defined energy levels.

            When the atoms are brought close to each other, they begin to influence each other due to mutual interaction between them. This mutual interaction is more in case of outer shell than the inner shells.

            If we take silicon as an element which is a semi-conduct with electronic configuration of 15² 25² 2P⁶ 35² 3P². Since the crystal contains at least 1023 atoms per unit volume so there may be at least 1023 sub-levels which are created. These sublevels lie very close to each other and hence they are known as the energy band.

The variation of energy levels w.r.t. inter-atomic separation is as explained below:

• When separation is d₁, the electrons in the outer-most shell of one atom do not interact with the electrons in the atoms the other due to large separation so, at this stage, each atom has its own specified energy level.
• When separation is d₂, the inter-atomic separation is less, so the electrons in the outermost shell start interacting with another due to which the modification of levels start (3s and 3p) levels, but still at this stage, energy gap occurs.
• When separation is d₃, inter-atomic separation reduces further and the independent 3s and 3p levels disappear. They are now distributed in a continuous manner, hence forming an energy band.
• When separation is r, the atoms are at there actual inter-atomic separation due to which the energy band splits up into two levels with a forbidden energy gap between them.

The lower filled band is known as the valence band is known as the valence band whereas the upper empty band is known as conduction band.

            On the basis of this energy band theory. solids are classified as:

(i)                 Conductor: –In conductor, the valence band and the conduction band overlaps i.e. no energy gap or there is almost negligible energy gap. For example: aluminum, gold etc.

(ii)               Insulator: – They have a large energy gap varying from 3eV to 7eV between conduction band and the valence band. For example: in case of diamond it is 6eV.

Semi-Conductors: – The energy gap between conduction band and valence band is approximately 1eV. For example : in Si, it is 1.1eV and in Ge is 0.7eV.

Thus this the concept of energy band or energy band gap in solids.