Within any material, there are two distinct energy bands in solids. These bands are the Valence energy band and the Conduction energy band and these bands are separated by an energy gap which is known as the forbidden energy gap.
Various types of energy bands in solids:
Conduction band or conduction energy band is the band that contains conduction electrons. The conduction band occupies the lowest energy levels.
The outermost shell of an atom is called a valence shell and the electron in the outermost shell is known as valence electron. These valence electrons contain energy levels and so form a valence energy band or valence band. These valence electrons are not tightly held to the nucleus.
The energy gap or forbidden energy gap is the gap between the conduction band and the valence band. This energy gap is not similar for conductors, semiconductors, and insulators. The energy gap doesn’t have any energy that’s why no electron normally exists in this band.
The energy gap plays a crucial in the distinction between conductors, semiconductors, and insulators. There is no forbidden gap in case conductors, a narrow forbidden gap (about 1 eV) for semiconductors, and a wide forbidden (more than 5 eV) in case of insulators.
Energy band in Conductors
Those materials which allow current to flow easily through them eg. Gold, copper, silver, aluminium all these metal are good conductor of electricity.
In conductors, There is no energy gap between the conduction band and valence band, and both bands overlap each other. This is why when a small amount of external energy is applied the electron moves from the valence band to the conduction band easily.
An electron in the conduction band experiences an almost negligible amount of nuclear attraction. In the conduction band electrons moves freely and those electrons are known as free electrons. These free electrons are available in large numbers in the conductor.
Energy band in Semiconductors
Semiconductors are those materials that have electrical conductivity between conductors and insulators. Silicon, germanium, etc. are examples of semiconductors.
At a temperature of absolute 0, the valence band of semiconductors is full and there are no electrons in the conduction band. However, in semiconductors, both bands are close to each other (around 1eV apart).
If external energy is applied electrons in the valence band can easily move to the conduction band. But the applied energy must be greater than the forbidden energy gap EG. If the applied energy gap is less than the forbidden energy gap, an electron can not move to the conduction band because no permissible energy levels exist between the two bands.
But energy is applied by heat at room temp. is sufficient to lift the electrons from the valence band to the conduction band. Thus, semiconductors are capable of conduction electric currents even at normal ambient temperatures.
For silicon forbidden energy gap is 1.1 eV and for germanium, it is 0.72 eV.
Energy band in Insulator
In layman’s terms, insulators are the materials that don’t allow an electric current to pass through them i.e insulators are poor conductors of electricity. Insulators have high resistivity and low conductivity. Wood, glass, etc. are some examples of insulators.
In insulators, there is no electron in the conduction band and the valence band is filled. The energy gap is very high (more than 5eV) in insulators that at room temperature the energy supplied to the electrons by externally applied field is too small to carry an electron from valence band to conduction band. Therefore, conduction in insulators is impossible.
That’s all about energy bands in solids (conductor, semiconductor, and insulator) and various types of energy bands in solids.