Fermi energy and quasi-Fermi energies in pn-junctions

In summary, the equation EFp=EFn-eVd=EF is a result of the fact that at thermal equilibrium, the diffusion potential, Vd, must be equal to the difference between the quasi-Fermi energies of the p-type and n-type regions. This is because when there is a difference between the two energies, carriers will diffuse across the junction until there is no longer a difference, i.e. until Vd= EFn-EFp. At this point, the Fermi energy in both regions will be equal, and hence EF = EFn = EFp.
  • #1
Skovgaard
1
0
Hi,

Considering a pn-junction at thermal equilibrium, why does following count

EFp=EFn-eVd=EF,

where EFp and EFn are the quasi-Fermi energies in the neutral p- and n-type regions before equilibrium is established, with EFn>EFp, EF is the Fermi energy after equilibrium is established, and Vd is the diffusion potential? What I do not get, is how EFp can be equal to EF.
 
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  • #2
This equation is a result of the fact that at thermal equilibrium, the diffusion potential, Vd, must be equal to the difference between the quasi-Fermi energies of the p-type and n-type regions. This is because when there is a difference between the two energies, carriers will diffuse across the junction until there is no longer a difference, i.e. until Vd= EFn-EFp. At this point, the Fermi energy in both regions will be equal, and hence EF = EFn = EFp. Hope this helps!
 

1) What is Fermi energy in a pn-junction?

Fermi energy, also known as the Fermi level, is the energy level at which the probability of finding an electron is 50% in a semiconductor material. In a pn-junction, it represents the energy at the interface between the p-type and n-type regions.

2) How does the Fermi energy change in a pn-junction?

In a pn-junction, the Fermi energy shifts towards the higher energy side due to the diffusion of majority carriers from one region to the other. This results in the formation of a built-in potential, which establishes an electric field across the junction.

3) What is the significance of quasi-Fermi energies in a pn-junction?

Quasi-Fermi energies are introduced in a pn-junction to account for the energy levels of both electrons and holes. In equilibrium, the quasi-Fermi energy levels for both the p-type and n-type regions are equal to the Fermi energy. However, under non-equilibrium conditions, they can have different values and are used to describe the carrier concentrations in each region.

4) How are quasi-Fermi energies affected by external biasing in a pn-junction?

When a pn-junction is biased, the quasi-Fermi levels in each region shift towards the direction of the applied bias. This results in an increase in the majority carrier concentration in one region and a decrease in the other, leading to a flow of current through the junction.

5) Can the quasi-Fermi energies be controlled in a pn-junction?

Yes, the quasi-Fermi energies can be controlled by external factors such as temperature, doping levels, and biasing. By adjusting these parameters, the quasi-Fermi levels can be manipulated to achieve specific carrier concentrations and therefore, control the behavior of the pn-junction.

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