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# Classification of Materials: What is Energy Band Theory?

Plotting the available energies for electrons in the materials, is a good approach to visualize the differences between conductors, insulators and semiconductors. The possible energy states form bands rather than distinct energies, as in the case of unbound atoms. The presence of electrons in the conduction band is critical to the conduction process. In insulators, the electrons in the valence band are separated from the conduction band by a large gap; in conductors, such as metals, the valence band overlaps the conduction band; and in semiconductors, the valence and conduction bands are separated by a small enough gap that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically. Energy band theory classifies the materials into conductors, insulators and semiconductors, according to their energy levels separation and electrical properties.

## Density of energy states

The density of states is the number of distinct states that electrons are allowed to occupy at a given energy level, i.e. the number of electron states per unit volume per unit energy. This function determines bulk properties of conductive substances such as specific heat, paramagnetic susceptibility, and other transport phenomena. DOS calculations can be used to calculate the general distribution of states as a function of energy in semi-conductors, as well as the spacing between energy bands.

## What is energy band theory?

The energy band theory is used to describe processes and effects in solid crystals when they are subjected to electromagnetic fields. The theory of a valence electron travelling in a periodic potential field of a crystalline lattice is known as energy band theory. A discrete energy spectrum indicates that single atoms can only occupy discrete energy levels. In a non-excited state, some of these energy levels are filled with electrons.  Part of these levels can be occupied only when electrons are excited.

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## What are energy bands?

The arrangement of molecules in gaseous substances is spread out and not so near to each other. The molecules in liquids are closer together. However, because the molecules in solids are much closer, the atoms of molecules tend to travel into the orbitals of nearby atoms. As a result, when atoms collide, their electron orbitals overlap. The intermixing of atoms in solids results in the formation of numerous bands of energy levels. These groups of energy levels are referred to as energy bands.

## Formation of energy Bands

The electrons in each orbit of an isolated atom have a specific amount of energy. However, in solids, the energy level of the outermost orbit electrons is influenced by the atoms nearby. The electrons in the outermost orbit encounter an attractive attraction from the neighboring atomic nucleus when two isolated charges are brought close together. Because of this, electron energies will not be at the same level and electron energy levels will be modified to a value that is greater or lower than the original energy level of the electron. The energy levels of electrons in the same orbit differ. The term energy band refers to the grouping of these various energy levels. However, the energy of inner orbit electrons is unaffected by the existence of nearby atoms.

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## Classification of Energy Bands

### What is valence band?

Valence electrons are the electrons in the outermost shell. The valence electrons create an energy band known as the valence band, which has a number of energy levels. The occupied energy in the valence band is the highest. Valence band can be completely filled or partially filled and can never be empty.

### What is conduction band?

Because the valence electrons are not strongly bound to the nucleus, a few of them exit the outermost orbit and become free electrons even at ambient temperature. Free electrons are known as conduction electrons because they transmit current in conductors. The conduction band has the lowest occupied energy levels and contains conduction electrons. Conduction band can either be empty of partially filled.

### What is forbidden energy gap?

The prohibited gap is the space between the valence band and the conduction band. The forbidden gap has no energy and no electrons, as its name implies. The valence band electrons are securely bound to the nucleus if the forbidden energy gap is higher. We’ll need a certain amount of external energy to fill the restricted energy gap. If some electron is provided by sufficient energy to cross this forbidden gap, then it will move from valence band to conduction band.

## Classification of materials

The distance between valence band and conduction band decides the name of material, which impact its electrical properties too. This is explained by energy band theory. According to energy band theory, there are three classifications on this basis, given below:

### What are conductors?

Conductors are the materials which have plenty of free electrons as their valence and conduction band overlap each other. So definitely there is no hurdle for electrons to move from valence to conduction band. They include most of metals as they can conduct. Gold, aluminum, silver and copper are all conductors, allowing an electric current to flow through them. Because there is no forbidden gap between the valence band and the conduction band, the two bands overlap. At normal temperature, there are a lot of free electrons available.

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### What are insulators?

The materials in which there is large band gap (usually 7 eV) between valence band and conduction band are known as insulators. They cannot conduct current as electrons cannot cross forbidden energy gap. Insulators include things like glass and wood. These materials prevent electricity from passing through them. They have a low conductivity and a high resistance. Most solids are insulators.

While doping insulators can significantly alter their optical properties, it is insufficient to overcome the huge band gap to make them good electrical conductors. Doping semiconductors has a significantly greater impact on their electrical conductivity and is the foundation for solid state electronics.

### What are semiconductors?

These are the materials whose electrical properties are in between conductors and insulators. They have smaller band gap that can be crossed by electrons if sufficient energy is provided. At room temperature, semiconductors behave like insulators, but if they are heated or energy in any other way is provided, they become conductors. Germanium and Silicon are the best materials for electrical characteristics that fall in between semiconductors and insulators. The conduction band is empty and the valence band is totally filled in the energy band diagram of a semiconductor, but the forbidden gap between the two bands is very small, around 1eV. The forbidden gap in Germanium is 0.3 eV, while in Silicon it is 0.7 eV. As a result, semiconductors offer low conductivity as compared to conductors.

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