Conduction band

From ArticleWorld

Conduction band refers to the area across which valence electrons move in order to accommodate a higher energy state. This movement or jumping is essential for the production of electric current.

Background

Electrons in an atom are distributed in various shells and are allowed to possess definite energy values. The valence electrons occupy the highest level of energy, but still higher levels of energy can be obtained when these electrons absorb a certain amount of outside energy. The electrons near the outer edge of atoms close to each other are disturbed by neighbouring atoms, resulting in disturbances in their energy levels. The energy levels in an atom can be understood as a system of bands, as explained by the band-structure theory. The lower energy levels corresponding to inner shells do not exhibit this nature; it is only the valence electrons that appear in bands.

How conduction works

Bands are occupied by the valence electrons and the levels above only. When the valence band electrons have sufficient energy to jump a so-called ‘forbidden gap’ and onto the conduction band, an electronic current is possible. This happens after an electric field is applied to the semiconductor crystal. Higher bands, above the forbidden gap, are almost continuously available for the conduction of electricity. Electrons are free to accelerate under an applied electric field when in this region. Thus, the band just above the forbidden gap is called the conduction band.

An intrinsic or pure semiconductor requires electrons or holes for conduction to take place. Since ‘holes’ are simply electron deficient regions in the semiconductor atoms, it is the electrons which are the actual carriers of electronic current. This makes it necessary for the electrons to gain enough energy to get energised and jump the band gap. Energy can be acquired by electrons after they are heated to a certain temperature, or if light of a certain value of intensity is focussed on it. Extrinsic semiconductors contain impurities called dopants which are added in order to provide charge carriers, namely electrons or holes in order to boost the conductivity of a semiconductor.

Probability of an electron jumping

The Fermi-Dirac formula is used to find out the probability that an electron will get energised. The probability is directly proportional to , where Eg stands for the energy of the band gap, k stands for Boltzmann’s constant and T denotes the temperature of the semiconductor material.