- When atoms bond together to form a solid, the orbit of an electron is influenced not only by the charges in its own atom but by nuclei and electrons of every atom in the solid. Since each electron occupies a different position inside the solid, no two electrons can have the same pattern of surrounding charges. Now in the solid, there are millions of electrons belonging to the first orbit of the atoms in the solid. Each of them have different energies and all these closely spaced energy levels differing only slightly in energy, .form a cluster or band. Similarly the second orbit and higher orbit electrons form bands. Hence the energy-band diagram of a solid (silicon) is shown.
- Although the third shell of an isolated silicon atom is not completely filled (it has only four electrons while it could have accommodated up to eight) the third energy band shown above is completely filled. This is so because each atom positions itself between four other atoms and each of the neighbours share and electron with the central atom. Now the valence band is filled, meaning all the permissible energy levels are occupied by electrons.
An additional band called the conduction band is shown above the valence band. This represents the next larger group of permissible energy levels. The gap between the conduction and valence band is called the forbidden energy gap. This is because there are no permissible energy levels in this region. An electron in the valence band can be lifted to the conduction band only if energy greater than or equal to this band gap energy is provided to the solid. Once they reach the conduction band, these electrons experience negligible nuclear attraction and move randomly throughout the solid. Hence they are called free electrons.
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