Band theory explain.The properties of solid substance
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Band theory, in solid-state physics, theoretical model describing the states of electrons, in solid materials, that can have values of energy only within certain specific ranges. The behaviour of an electron in a solid (and hence its energy) is related to the behaviour of all other particles around it.
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1.Band theory,
In solid-state physics, theoretical model describing the states of electrons, in solid materials, that can have values of energy only within certain specific ranges. The behaviour of an electron in a solid (and hence its energy) is related to the behaviour of all other particles around it. This is in direct contrast to the behaviour of an electron in free space where it may have any specified energy. The ranges of allowed energies of electrons in a solid are called allowed bands. Certain ranges of energies between two such allowed bands are called forbidden bands—i.e., electrons within the solid may not possess these energies. The band theory accounts for many of the electrical and thermal properties of solids and forms the basis of the technology of solid-state electronics.
The band of energies permitted in a solid is related to the discrete allowed energies—the energy levels—of single, isolated atoms. When the atoms are brought together to form a solid, these discrete energy levels become perturbed through quantum mechanical effects, and the many electrons in the collection of individual atoms occupy a band of levels in the solid called the valence band. Empty states in each single atom also broaden into a band of levels that is normally empty, called the conduction band. Just as electrons at one energy level in an individual atom may transfer to another empty energy level, so electrons in the solid may transfer from one energy level in a given band to another in the same band or in another band, often crossing an intervening gap of forbidden energies. Studies of such changes of energy in solids interacting with photons of light, energetic electrons, X-rays, and the like confirm the general validity of the band theory and provide detailed information about allowed and forbidden energies.
A variety of ranges of allowed and forbidden bands is found in pure elements, alloys, and compounds. Three distinct groups are usually described: metals, insulators, and semiconductors. In metals, forbidden bands do not occur in the energy range of the most energetic (outermost) electrons. Accordingly, metals are good electrical conductors. Insulators have wide forbidden energy gaps that can be crossed only by an electron having an energy of several electron volts. Because electrons cannot move freely in the presence of an applied voltage, insulators are poor conductors. Semiconductors have relatively narrow forbidden gaps—which can be crossed by an electron having an energy of roughly one electron volt—and so are intermediate conductors.
2.Band Theory of Solids
A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands 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.
An important parameter in the band theory is the Fermi level, the top of the available electron energy levels at low temperatures. The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties.