Energy Bands: Formation, Categorization and Types

Energy bands are atoms which are close to each other due to changes in the energy levels of electrons within their shell. In solids, molecules are arranged in a way in which atoms tend to move into the orbits of the atoms which are close to them. Therefore, orbits of electrons overlap when atoms come together. By intermixing atoms in the solid state, several bands of energy levels are formed. These energy levels are called Energy Bands.

Key Terms: Energy bands, Electrons, Valence Band, Atoms, Insulators, Conductors, Molecules, Forbidden energy gap, Semiconductors

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How are Energy Bands Formed?

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Molecules in gaseous substances are arranged in a manner in which they are not very close to each other. While in the liquid substances, they are closer to one another. The electrons in every orbit contain a specific amount of energy in a single atom. In solids, all the atoms are closely packed with each other. This affects the energy level of electrons of outer orbit because of the surrounding atoms. 

What Are Energy Bands?

When two isolated atoms are brought together, the electrons in the outer orbit of one of the atoms experiences a force from the atom nearer to it. Because of this process, the energies of the electrons change. This means that the energy levels will not be the same. The value of the electrons is changed to a higher or lower energy level than the original energy level of the electron.

Energy levels of the electrons in the same orbit are different every time. When you group these varied energy levels the energy band is formed. Energy levels of electrons of inner orbits are not affected by the atoms which are nearer to them.

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Categorization of Energy Bands

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Bohr’s hypothesis states that the shell of each atom contains a different amount of energy at different levels. Electron communication between the inner and outer shell is the main focus of this theory. So, according to this theory the energy bands are classified in three different types. The categorization of the energy bands are as follows: -

Valence Energy Band

The electrons travel in the atom at a fixed energy level. However, the electron in the inner shell possesses greater energy while compared to the electron in the outermost shell. The electrons which are found in the outermost shell are called Valence Electrons. These valence electrons are usually formed by a series of energy levels which in turn makes the valence band. This valence band is known to have the highest level of occupied energy.

Conduction Band Free Electrons

Conduction Energy Band

The electrons in the outer shell which are also called valence electrons are connected loosely to the nucleus at a normal temperature. Some of these valence electrons can depart the energy band freely. 

In this setting, these valence electrons flow towards the atoms which are near them, these electrons are known as free electrons. They are also called Conduction Electrons. The conduction electrons conduct the current flow within the conductor. The band in which the conduction electrons are included is called conduction energy band. The energy occupied by the conduction band is lower.

Forbidden Energy Gap

There is a space between the conduction and valence band, this space is prohibited. This prohibited space is known as the forbidden gap. This band lacks energy which in turn doesn’t allow this band to exist at all. This results in zero electron flow in this band. This forbidden gap allows the electron to travel from the valence to the conduction state. If this forbidden gap is larger, the electrons in the valence band get closer and tightly packed to the nucleus. Sometimes, external force is required to push out the electrons from this band.

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Types of Energy Bands

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Different types of energy bands are as follows: 

Conductors

Conductors are made up of the material in which the valence band or the forbidden energy gaps vanish. Then the conduction band becomes very close to the point where it is overlapped partially. There is a high number of free electrons available at room temperature. The most common examples of conductors are Gold, Aluminium, Gold and Copper.

The most crucial thing about conductors is the forbidden energy gap. The valence and conduction energy bands will probably get entangled if the energy gap is not there. So many free electrons are available for conduction. The conduction grows when the number of voltages increases.

Features of conductors are as follows:

1. There is no prohibited energy gap in a conductor.
2. The conduction and the valence bands overlap in the conductors.
3. The conduction rises when the voltage gets increased.
4. For power transmission there are high numbers of free electrons.

Types of Energy Bands

Insulators

The substances which do not conduct electricity and do not allow the electricity to pass through them are known as Insulators. The prohibited energy gap is big enough to not allow the electricity to pass through them. Insulators are materials like rubber and wood etc.

The features of the insulators are as follows:

1. The prohibited gap is wide enough in the insulators which has a value of 10 eV.
2. Sometimes conduction occurs in some insulators when the temperature increases.
3. Electrons are strongly packed together in the valence band.

Semiconductors

The materials which have conductivity between conductors and the insulators are called Semiconductors. In semiconductors, electricity can only be conducted if there is energy which is applied externally, because the prohibited energy gap is tiny. Germanium and silicon are some of the examples of semiconductors.

Properties of semiconductors are as follows:

1. The prohibited energy gap in semiconductors varies depending upon the material. The prohibited energy gap for silicon is 1.1Ev while on the other hand for Germanium it is 0.7eV.
2. The energy gap is minimal in the semiconductors.
3. As the temperature increases the connectivity rises in the semiconductors.
4. Semiconductors do not possess strong conductivity.
5. Semiconductors do not have good insulating properties.

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Things to Remember

• The number of atoms which are close to each other and the number of electrons that interact with each other in solids are known as Energy Bands.
• Bohr’s hypothesis describes that the shell of each atom contains a different amount of energy at different levels. Electrons communicate between the inner and outer shell which is the main focus of this theory.
• The valence electrons should have the same energy as the energy gap to move from the valence to the conduction band.
• There are three major classifications in the energy bands which are – Valence band, Conduction band, Forbidden energy band.
• Conductors, Insulators and Semiconductors are the three major types of energy bands.

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