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While studying about band theory of semiconductors, I observed that when the electrons were excited from the valence band to the conduction band, they left behind holes in the valence band. From my existing knowledge, I believe that the valence electrons alone occupy the valence band which tells me the valence band is negative. For the holes to exist, the valence band has to be neutral. So, why is the valence band neutral?

If my reason for the valence band to be negative was wrong, I would like to know the reason.

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I think if you look at the overall structure is neutral. I don't think that has much sense take valence band it self... – G M Apr 5 '14 at 17:47

Using an organic example, as you move from ethylene to butadiene to hexatriene, you have 1, 2 and 3 bonding and antibonding pi molecular orbitals respectively. If you continue this chain extension and wind up with a very large polyene, the energy difference between the all of the bonding (or antibonding) orbitals becomes small and instead of having discreet molecular orbitals, you can imagine them "fuzzing" out into a "band" of molecular orbitals. All of the bonding orbitals would become what is termed the valence band, while the antibonding orbitals would comprise the conduction band. Just as ethylene is neutral, so is the large polyene. If you excite one of ethylene's pi electrons to the antibonding molecular orbital you have created an excited state which is still neutral. Scientists working with the solid state would use different terms to describe this process. They would say that the antibonding orbital contains 2 holes and the boding orbital 2 electrons. When the electron is promoted to the excited state a hole is also promoted (holes have lower energy the further they are from the nucleus) from the antibonding orbital to the boding orbital. Just like the ground state, the excited state molecule is still neutral, the electron and hole are just in different orbitals from where they started.

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