Band theory is introduced in order to account for some properties of semiconductors. Have you seen any crystal stone? We know that in an atom there are electrons and some amount of energy in their definite energy levels.
A large number of atoms in a crystal stone is closer to each other and more those electrons interact with themselves. The energy level of electrons in their shell is only caused because of the modification in their energy levels. The most important feature of Energy Bands is that the energy states for electrons are continuous in varied ranges. Thus, we can say that the energy level of an atom vary in valence bands and in conduction bands.
Since the electrons interact with other electrons, the energy associated with a material is altered so, the electrons are associated with a band of energy levels. The collection of energy levels which are partially or wholly filled are called valence band, and they are never empty. The collection of energy levels of the free electron which move freely around the material are called conduction band. There is an extra energy required to the conduction band by the valence electrons to move, which is termed as Forbidden energy.
Depending on the material the energy band theory is classified as:
Gold, Aluminum, Silver, Copper, all these metals allow electric current to flow through them. There are two figures which depict that there is a small energy gap between the two bands as valence band is completely filled and conduction band is partially filled.
The other figure shows that conduction band is empty and the valence band is completely filled. There is an Energy Bands which is partially filled in both the situations. Therefore, the metals will conduct electricity even if a small amount of electric field is applied.
Glass and wood are the examples of the insulator; these substances do not allow electricity to pass through them. They have high resistivity and very low conductivity.
In the figure shown, the valence band is completely filled and the conduction band is empty thus the forbidden gap is vast and it is greater than 3eV. Hence, electrical conduction is impossible in these materials.
Germanium and Silicon are the most preferable material whose electrical properties lie in between semiconductors and insulators. The energy band diagram of semiconductor is shown where the conduction band is empty and the valence band is completely filled but the forbidden gap between the two bands is very small that is about 1eV. For Germanium, the forbidden gap is 0.72eV and for Silicon it is 1.1eV. Thus, semiconductor requires small conductivity.
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