Why Is the Fermi Energy Constant Across a PN Junction at Equilibrium?

[Swapnil]

Why is it that when no voltage is applied across the pn junction, the fermi energy is the same for both regions?

 

[Gokul43201]

If there are no other forces (gradients in thermal or mechanical potentials) acting on the particles (charge carriers), then a difference in chemical potentials will give rise to a particle current. So, the system can not be in equilibrium unless the chemical potentials are equal.

 

[marlon]

Why is it that when no voltage is applied across the pn junction, the fermi energy is the same for both regions?

Because that system is in thermodynamic equilibrium after contact between the p and n type SC is made. There is no net charge transfer over the interface and this is expressed by a fermi level (which is in fact the chemical potential in the case of a SC and insulator) that is horizontal and equal through the entire interface.

marlon