Energy band diagram of solids

Energy band diagram of solids

In an isolated atom, the electronic energy levels are widely separated and are far apart and the energy of the electron is decided by the orbit in which it revolves around the nucleus. However, in the case of a solid, the atoms are closely spaced and hence the electrons in the outermost energy levels of nearby atoms influence each other. This changes the nature of the electron motion in a solid from that of an isolated atom to a large extent.

The valence electrons in an atom are responsible for the bonding nature. Let us consider an atom with one electron in the outermost orbit. It means that the number of valence electrons is one. When two such atoms are brought close to each other, the valence orbitals are split into two. Similarly the unoccupied orbitals of each atom will also split into two. The electrons have the choice of choosing any one of the orbitals as the energy of both the orbitals is the same. When the third atom of the same element is brought to this system, the valence orbitals of all the three atoms are split into three. The unoccupied orbitals also will split into three.

In reality, a solid is made up of millions of atoms. When millions of atoms are brought close to each other, the valence orbitals and the unoccupied orbitals are split according to the number of atoms. In this case, the energy levels will be closely spaced and will be difficult to differentiate the orbitals of one atom from the other and they look like a band as shown in Figure 9.2. This band of very large number of closely spaced energy levels in a very small energy range is known as energy band.

The energy band formed due to the valence orbitals is called valence band and that formed due to the unoccupied orbitals to which electrons can jump when energised is called the conduction band. The energy gap between the valence band and the conduction band is called forbidden energy gap. Electrons cannot exist in the forbidden energy gap.

The representation of the valence band and conduction band is shown in Figure 9.2(a). EV represents the maximum energy of the valence band and EC represents minimum energy of the conduction band. The forbidden energy gap, E_g = E_C– E_v. The kinetic energy of the electron increases from bottom to top (near the nucleus to the farthest) and the potential energy decreases indicating that the electrons in the orbitals closer to the nucleus are bound with large potential energy. Hence, the electrons closer to nucleus require a lot of energy to be excited. The electrons in the valence band are less bound to the nucleus and can be easily excited.

The energy levels of the orbiting electrons are measured in electron volts, (eV).