The Semiconductor Junction (a.k.a. the p-n or n-p Junction)

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Imagine a p-type and n-type semiconductor are brought into contact. Realistically it would be the same piece of semiconductor with one part doped with acceptor atoms and another with donor atoms but the idea of bringing two pieces into contact will be acceptable for now. The n-type piece will have an excess of electrons and the p-type an excess of holes. By diffusion the n-type electrons will begin to move over to the p-type side and p-type holes will move to the n-type side. These electrons and holes will begin to combine as excess electrons move back down to the conduction band. This will give the n-type region an excess of holes and therefore a positive charge while the p-type region gets an excess of electrons and so a negative charge, creating a potential V0. This potential will resist the flow of electrons from n-type to p-type and so will eventually stop the flow when sufficient electrons have passed across. The energy levels in the n-type region are then decreased by -eV 0 due to their attraction to the new positive charge and increased in the p-type region by eV0 due to the repulsion to the new negative charge.
The region where the potential changes is known as the depletion region and is typically on the order of 10 micrometers. The change in the potential causes and electric field directed from the n-type side to the p-type side. This will then push any stray holes in the n-type side to the p-type side and electrons from p-type to n-type.
The Fermi Energy is the same across the junction while still being near the valance band on the p type side and near the conduction band on the n-type side as a consequence of the energy level shifting due to the potential. This only applies when the system is in dynamical equilibrium and so when no external potential difference is applied. The state of equilibrium implies that the current of electrons and holes diffusing from the p side to the n side is equal to the diffusion current from the n side to the p side, so that no resultant current flows.

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