Energy Bands Description
In gases, the arrangement of molecules is not at all close, that is, they are far away from each other, and are loosely packed. The molecular arrangement in liquids is moderate, that is, the molecules are a little far away from each other. When it comes to solids, the molecules are so tightly packed or arranged that the electrons (a sub-atomic particle with an electric charge of negative 1) tend to move towards the orbitals of the neighbouring atoms. Consequently, the electron orbitals overlap as and when the atoms come together. Because of the intermixing of atoms in the substances of the solid-state, there will be a formation of energy bands, instead of the single energy levels. The set of energy levels, which are closely or tightly packed, are what we call the Energy Bands.
Classification of Energy Bands
Valence Band
Although the electrons move in the atoms in certain energy levels, the energy of the electrons present in the innermost shell is higher than the energy of the electrons present in the outermost shell. Valence electrons are the electrons, which are present in the outermost shell. The valence electrons contain a series of energy levels and form an energy band known as the valence band. The valence band is the band, which has the highest occupied energy.
Conduction Band
The valence electrons are not held tightly or firmly to the nucleus, due to which, even at room temperature, a few of the valence electrons leave the valence band to become free. They are referred to as the free electrons because of the fact that they tend to move towards the neighbouring atoms. The free electrons conduct the current in the conductors and are therefore known as the conduction electrons. The conduction band is the one that contains the conduction electrons and has the lowest occupied energy levels.
Forbidden Energy Gap
The forbidden energy gap refers to the gap between the valence band and the conduction band. As the name suggests, the forbidden energy gap has no energy as a result of which no electron stays in this energy band. While going to the conduction band, the valence electrons pass through the forbidden energy gap. If the forbidden energy gap is greater, then the valence band electrons are tightly bound or firmly attached to the nucleus. For pushing the electrons out of the valence band, we require some amount of external energy equal to the forbidden energy gap.
The figure given below shows the conduction band, valence band, and the forbidden energy gap. Based on the size of the forbidden energy gap, the conductors, semiconductors, and insulators are formed.
Conductors
Conductors are the substances or materials that conduct electricity as they allow electricity to flow through them. The forbidden energy gap disappears in the conductors, as the conduction band and the valence band come close to each other and overlap. Copper, gold, and silver are a few examples of conductors. The figure given below shows the structure of energy bands in conductors.
The Characteristics of Conductors are as Follows:
There is no forbidden energy gap in a conductor.
The valence band and the conduction band overlap in conductors.
There are a high number of free electrons available for the conduction of electricity.
With a slight increase in voltage, there is an increase in the conduction as well.
No concept of hole formation is there because the continuous flow of electrons contributes to the current produced.
Insulators
Insulators are the substances or materials that don't conduct electricity as they don't allow electricity to flow through them. The forbidden energy gap in the insulators is large enough due to which the conduction of electricity can't take place. Rubber and wood are a few examples of insulators. The figure given below shows the structure of energy bands in insulators.
The Characteristics of Insulators are as Follows:
The forbidden energy gap is large enough in insulators with a value of 10eV.
The electrons in the valence band are tightly bound or firmly attached to atoms.
Some insulators might show conduction with an increase in the temperature.
Semiconductors
Semiconductors are substances or materials having conductivity between the conductors and the insulators. In semiconductors, the forbidden energy gap is small, and the conduction of electricity will take place only if we apply some external energy. Germanium and silicon are a few examples of semiconductors. The figure given below shows the structure of energy bands in semiconductors.
The Characteristics of Semiconductors are as Follows:
The forbidden energy gap is small in a semiconductor.
For Germanium (Ge), the value of the forbidden energy gap is 0.7eV, and for Silicon (Si), it is 1.1eV.
The conductivity of semiconductors increases with the rise in temperature.
Semiconductors are neither a good conductor or an insulator.
