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In brief, every atom consists of valance electrons i.e electrons in the outer most orbits. These are bound to the nucleus though with less force and they belong to a praticular atom. If sufficient energy is given to them, they will go to still higher orbits and are now called as free or conduction electrons. These electrons belong to the whole substance rather than the atom ( valance electrons are like pet dogs, while conduction electrons are like stray dogs
) The collection on all valance electrons in a substance will from what is called valance band while conduction electrons form a band called conduction band. There is a band energy gap between these bands. for the valance electrons to become conduction electrons, they have to acquire the energy equal to the band energy gap between the valance and conduction bands. Hope the concept is clear. Cheers
The Conduction Band:
The conduction band is the upper band of allowed states. When it is drawn it is represented by a line labeled by Ec which represents the lowest possible energy state in the conduction band.
This band is usually empty, it contains few or no electrons since energy is required for them to get there from the valence band. Electrons in the conduction band are free to move about the crystal, thus the name conduction band.
If an electron does manage to get to the conduction band, it resides there for mere fractions of a second (an average lifetime). When it loses its energy it drops back down to the valence band emitting its energy as heat, light or by transferring it to another electron.
In solids, the valence band is the highest range of electron energies where electrons are normally present at absolute zero.
In semiconductors and insulators, there is a band gap above the valence band, followed by a conduction band above that. In metals, the conduction band has no energy gap separating it from the valence band (basically, this is correct only for semimetals. All solids have forbidden energy levels between the energy bands). The rest of this article refers to the valence band in semiconductors and insulators.
Semiconductor band structure
See electrical conduction and semiconductor for a more detailed description of band structure.
Semiconductors and insulators owe their low conductivity to the properties of the valence band in those materials. It just so happens that the number of electrons is precisely equal to the number of states available up to the top of the valence band. There are no available states in the band gap. This means that when an electric field is applied, the electrons cannot increase their energy (i.e., accelerate) because there are no states available to the electrons where they would be moving faster than they are already going.
There is some conductivity in insulators, however. This is due to thermal excitation—some of the electrons get enough energy to jump the band gap in one go. Once they are in the conduction band, they can conduct electricity, as can the hole they left behind in the valence band. The hole is an empty state that allows electrons in the valence band some degree of freedom.
It is a common misconception to refer to electrons in insulators as "bound"—as if they were somehow attached to the nucleus and could not move. Electrons in insulators are free to move. They are also delocalized, having no well-defined position within the sample.
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