The definition of an energy band is the number of atoms within a substance that can be as close to each other as possible and at the same time that many electrons will interact with each other. The energy level of electrons within their shell can be caused by changes in their energy levels.
The arrangement of molecules in gaseous substances is not close. In liquids, the molecular arrangement is moderate. But, in solids, the molecules are so closely arranged that the electrons in the atoms of the molecules move to the orbits of neighboring atoms. So when atoms come together the electron orbitals overlap.
In solids, atoms bond together to form bands of energy levels rather than single energy levels. These sets of energy levels, which are closely packed, are called energy bands.
Theory of Energy Band
According to this Bohr principle, each shell of an atom contains a different amount of energy at unequal levels. This theory mainly gives details about the communication of electrons between the inner shell and outer shell. According to the theory of energy bands, energy bands are classified into three types which are as follows.
- Valence band
- Forbidden energy gap
- Conduction band
Valence Band
The flow of electrons within atoms occurs at certain energy levels, although the energy of electrons in the inner orbital is better than that of electrons in the outer orbit. The electrons present inside the outer shell are called valence electrons.
These electrons have a sequence of energy levels that form an energy band called the valence band. This band has maximum occupied energy.
Conduction Band
At room temperature, the valence electrons are loosely coupled towards the nucleus. Some of the valence electrons will leave the electron band freely. That’s why they are called free electrons because they flow to neighboring atoms.
These free electrons will conduct a flow of current within a conductor known as conduction electrons. The band in which the electrons are there is named the conduction band and the energy occupied in it will be less.
Forbidden gap
The gap between the valence band and the conduction band is called the forbidden energy gap. This band is barred without energy. Hence no electrons remain in this band. The valence electrons, while moving to the conduction band, pass through it.
If the forbidden energy gap is greater, it means that the valence band electrons are tightly bound to the nucleus. Now, to push the electrons out of the valence band, some external energy is required, which will be equal to the forbidden energy gap.
In the following diagram, the forbidden gap along the two bands is shown below. Semiconductors, conductors, and insulators are made on the basis of gap size.
Types of Energy Bands
Energy bands are classified into three types:
- Insulators
- Semiconductors
- Conductors
Insulators
The best examples of insulators are wood and glass. These insulators do not allow the flow of electricity through them. Insulators have extremely low conductivity and high resistivity. In an insulator, the energy difference is very high which is 7eV. The flow of electrons from the band causes the material to perform such that from the valence to conduction is inexcusable.
Characteristics of Insulators
The following are the characteristics of the insulator:
- The forbidden energy gap is huge.
- The value of the forbidden energy gap for an insulator will be 10eV.
- The valence band electrons are tightly bound to the atoms.
- The resistivity of a conductor will be in the order of 107 ohm-meter.
- For some insulators, as the temperature increases, they may show some conduction.
Semiconductors
Semiconductors are materials in which the forbidden energy gap is small and conduction occurs if some external energy is applied.
The best examples of semiconductors are silicon (Si) and germanium (Ge) which are the most commonly used materials. The electrical properties of these materials range between those of semiconductors as well as insulators. The following diagrams show the energy band diagram of a semiconductor wherever the conduction band can be empty and the valence band is completely filled, however, the forbidden difference between these bands is min which is 1eV. The forbidden gap of Ge is 0.72eV and that of Si is 1.1eV. Therefore, semiconductors require low conductivity.
Characteristics of Semiconductors
The following are the characteristics of the Semiconductors:
- The forbidden energy gap is very small.
- A semiconductor is really neither an insulator nor a good conductor.
- The forbidden gap for Ge is 0.7eV while that for Si is 1.1eV.
- The conductivity of a semiconductor will be in the order of 102 ohm-meter.
- As the temperature increases, the conductivity of the semiconductor increases.
Conductors
A conductor is a material in which the forbidden energy gap disappears as the valence band and conduction band becomes so close that they overlap.
The best examples of good conductors are aluminum, copper, and gold. The availability of free electrons at room temperature is huge. The energy band diagram of the conductor is shown below.
Characteristics of Conductors
The following are the characteristics of the Conductors:
- No forbidden gap exists in the conductor.
- There are plenty of free electrons available for conduction.
- The valence band and the conduction band overlap.
- There is no concept of the formation of holes because the constant flow of electrons contributes to the current.
- A slight increase in voltage leads to an increase in conductivity.