Why is Fermi Level constant across pn junction?

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SUMMARY

The Fermi level remains constant across a pn junction under thermal equilibrium due to the principles of thermodynamics and statistical mechanics. When there is a difference in occupancy at a specific energy level (E) on either side of the junction, the system is not in equilibrium, prompting electron movement that increases entropy and stabilizes energy. This behavior is essential for understanding charge carrier dynamics in semiconductor physics.

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chingkui
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I know this is true under thermal equilibrium, but I am not sure why. Why can't the Fermi level be a function of position relative to the pn-junction? How does that follow from thermodynamics/statistical mechanics?
 
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If you have (more than 50% occupancy at energy E) on one side and (less than 50% occupancy at energy E) at the other side, the system is not in equilibrium, and electrons can move from one side to another, increasing entropy and reducing the energy of the system.
 

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