Semiconductor Basics

The Energy Band Theory of Crystals

According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in which it revolves. Only discrete values of electron energies are possible. An electron cannot have any value of energy but only certain permissible values. No electron can exist at an energy level other than a permissible one.

Energy Band Diagram for an Isolated Hydrogen atom.

When atoms bond together to from a crystal, the simple diagram of Electron Energy is no longer applicable. In a crystal, 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 see exactly the same pattern of surrounding charges. As a result, the orbits of the electrons are different.

There are millions of electrons, belonging to the first orbit atoms in the crystal. Each of the them has different energy. Since there are millions of first orbit electrons, their energy levels differs only slightly and forms a band. So first energy band is formed by energy of electrons when they are in 1^st orbit,  second band if formed by energy of electrons in 2^nd orbit. In this way third band, fourth band, etc are formed.

Valence band : The Highest energy Band filled with valence electrons is called Valence band.      
Conduction band : The lowest unfilled allowed energy band next to valence band is called conduction band.

In semiconductors, only valence band and conduction band are important as electron of valence band are loosely bonded as they already have high energy and can transit to conduction band. The Gap between the top of valence band and bottom of conduction band is called forbidden gap or energy band gap or energy gap. No allowed energy level for electrons can exist. 
(For Silicon Eg=1.12 eV, and for Germanium Eg=0.72eV).

Energy Band Diagram for a Silicon Crystal.  Electron Configuration of Silicon atom =2,8,4.