Origin of Energy Band Formation in Solids

ORIGIN OF ENERGY BAND FORMATION IN SOLIDS

We know that when two atoms of equal energy levels are brought closer together, the original energy levels viz E₁ and E₂ splits each into two energy levels i.e., the K – shell energy E₁ splits into E₁¹ and E₁² similarly the L- shell energy E₂ splits into E₂¹andE₂² (Fig. 1.31 and Fig. 1.32). Now, when three atoms are brought closer together, the original energy levels viz E₁ and E₂ splits each into three energy levels viz E₁¹ , E₁² , E₁³ and E₂¹ , E₂² , E₂³  respectively as shown in fig 1.33. These type of transformations from the original energy levels into two (or) more energy levels is known as Energy Level Splitting.

Energy Bands

Therefore, if 'N' number of atoms of equal energy levels are brought closer to form a solid, then it forms a closely spaced contineous energy levels, so called energy bands as shown in fig 1.34.

Hence an energy band can be defined as, the range of energies possessed by an electron in a solid. The magnified view of the energy band which consists of a large number of very closely spaced energy levels is shown in Fig. 1.35.

Note: The overall range of energies from the lowest energy level to the highest energy level of a band is called the width of band. From Fig. 1.35, we can also see that the width of band increases gradually due to the increase of electrons from the inner most orbit towards the outer most orbit.

Inner Filled Bands, Valence Band and Coduction Band

During the formation of energy bands, the inner filled energy levels forms a energy band, called Inner filled bands.

Similarly the electrons in the outermost shells of atoms forms a energy band, called valence band. The valence band will be of completely filled (or) partialy filled with electrons, based on the type of the materials.

If an electron comes out from valence band for conduction, then they form an energy level corresponding to the energy band called conduction band as shown in Fig. 1.36.

Forbidden Band

While referring to energy bands, they are separated by small regions which does, not allow any energy levels. Such regions between the energy bands are called forbidden bands (or) forbidden energy gap E, as shown in Fig. 1.36.

Note: Though there are number of energy bands in solids, valence band (The range of energies possessed by valence electrons) and conduction band (The range of energies possessed by conduction electrons) are particularly important, because these valence electrons (or) so called free electrons are capable of electrical conduction, when it moves from valence band to conduction band.