Question 3: Describe the valence band, conduction band and forbidden energy gap with the help of energy level diagram.
Energy band theory
In a single isolated atom, electrons remain in definite energy levels. However, in solids, the atoms are close to each other and the energy levels of the outermost orbit electrons are affected by the neighboring atoms. The electrons of the outermost orbits of one atom experiences an attraction from the neighboring nuclei. The energy levels of electrons are changed into bands (i-e, groups) of closely spaced levels with large energy gaps between them. The spaces between these energy gaps are called forbidden energy states (or gaps) and the bands as permissible energy levels. In solids, large number of atoms and the energy levels divide into bands of closely spaced levels with large energy gaps between them. These energy states are discrete (separate) in nature but are so close to each other that they appear in the form of a continuous energy band.
Similarly, the energy levels of the inner orbit electrons are not much affected by the presence of the neighboring atomic nuclei.
Energy band theory of solids can explain many electrical and optical properties of solids.
Important energy bands in solids
Following are the important energy bands in solids. They are explained on the basis of energy level diagram.
- Valence band
- Conduction band
- Forbidden gap
Valence band: See the figure above showing energy levels of atoms in solid structure. The lower band is called valence band. In valence band the electrons are tightly bound to the nucleus of the respective atom and cannot move around in the body of the solid. This band may never be empty. However, in some cases, they may be partially filled.
The bands below valence band are completely filled with electrons and do not take part in any electrical activity.
Conduction band: This is the upper band; see figure. In this band, the electrons are freely moving about, causing the electric current when a potential difference is applied across the solid (conductor). This band may be empty, or partially filled.
When energy is supplied to the electrons in the valence band, they jump to the conduction band and are accommodated there.
Forbidden energy gap: Forbidden energy gaps are situated between valence and conduction bands. The electrons cannot stay in this region. Forbidden energy gap is much larger in case of insulator material.
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