As isolated atoms are brought together to form a solid, various interactions occur between neighboring atoms. The forces of attraction and repulsion between atoms will find a balance at the proper interatomic spacing for the crystal. In the process, important changes occur in the electron energy level configurations, and these changes result in the varied electrical properties of solids.
Energy Band diagram:
- In a solid, many atoms are brought together, so that the split energy levels form essentially continuous bands of energies.
- As an example, Fig. given below illustrates the imaginary formation of a silicon crystal from isolated silicon atoms.
- Each isolated silicon atom has an electronic structure 1s2 2s2 2p6 3s2 3p2 in the ground state.
- Each atom has available two 1s states, two 2s states, six 2p states, two 3s states, six 3p states, and higher states.
- If we consider N atoms, there will be 2N, 2N, 6N, 2N, and 6N states of type 1s, 2s, 2p, 3s, and 3p, respectively.
- As the interatomic spacing decreases, these energy levels split into bands, beginning with the outer (n = 3) shell.
- As the "3s" and "3p" bands grow, they merge into a single band composed of a mixture of energy levels.
- This band of "3s-3p" levels contains 8N available states.
- As the distance between atoms approaches the equilibrium interatomic spacing of silicon, this band splits into two bands separated by an energy gap Eg.
- The upper band (called the conduction band) contains 4N states, as does the lower (valence) band. Thus, apart from the low-lying and tightly bound "core" levels, the silicon crystal has two bands of available energy levels separated by an energy gap Eg wide, which contains no allowed energy levels for electrons to occupy.
- This gap is sometimes called a "forbidden band," since in a perfect crystal it contains no electron energy states.