Work function of a semiconductor is given by the energy required to take electron from Fermi level to vacuum level. Under illumination, Fermi level of a semiconductor is replaced by two quasi Fermi levels. How to calculate the work function then?
The work function has an relationship with the the illumination :the Fermi level décalés to the conduction band,the new work function ph=Evacc-EF0-∆EF
You may be interested in this paper:
S.K. Ravi et al, "Optical manipulation of work function contrasts on metal thin films"
Science Advances 02 Mar 2018: Vol. 4, no. 3, eaao6050 DOI: 10.1126/sciadv.aao6050
https://advances.sciencemag.org/content/4/3/eaao6050
In the n-Type semiconductor, the Fermi level lies closer to the conduction band because of a higher concentration of electrons than that of holes.
then the new work function is:
Φ= Evac.-EFo - ΔEF
Umberto Ravaioli Thank you for the paper. However, it describes the effect of illumination on metal- semiconductor contact, instead of a semiconductor.
Naceur Selmane Ashish Bhatt The equation mentioned by you implies that Fermi level moves downward (away from conduction band) on illumination. It will be helpful if you could share the reference.
ShuchI:
It looks like equation Ashish Bhatt has given is the change in work function before and after illumination where delta_Ef = Efn-Efp where Efn>Efp for an n type semiconductor, Efn is also closer to conduction band than Ef level before illumination, similarly Efp is closer to valence band. delta_Ef term arrives under illumination when you consider the work function due to Efn and Efp considering Evac being fixed. Before illumination, in an n-type semiconductor phi_work function = Evac-Ef where Ef is determined by initial majority n type doping concentration before excess carrier generation. I do not have any reference to support my claim but you can intuitively get the picture and the equation by drawing the band diagram before illumination and after illumination with quasi Fermi levels.
Sincerely,
Dr. Nabil Shovon Ashraf
Associate Professor
North South University
Dhaka, Bangladesh.
Dear all
The answer is to take the quasi-Fermi level of majority carriers (electrons) which tends to the equilibrium Fermi level at contacts. This is very near to the conduction band in n-type semiconductor. Contacts are usually assumed at equilibrium. I think you'd be interested in knowing the work function only there.
Under sustained illumination the device is in steady state instead of thermal equilibrium although contacts can be at equilibrium but when the illumination is spatially uniform, a single reference value to determine work function at contact, I do not know how it works.
Shuchi:
You may look into attached lecture on quasi Fermi level and its non equilibrium excess carrier generation kinetics.
Sincerely,
Dr. Nabil Shovon Ashraf
Associate Professor
Department of ECE
North South University
Dhaka, Bangladesh.
Shuchi Kaushik This is a rather old question but maybe this is still useful: Many doped semiconductor surfaces have the Fermi level pinned due to surface states (for example, in GaAs the Fermi level is pinned near the middle of the gap). This leads to band bending near the surface (similar to the bending in a pn-junction). Eluminating the surface leads to the so-called surface photovoltage effect (analogous to how shining light on a pn-diode/solar cell causes a voltage change). Hence, shining light onto a doped semiconductor surface increases/decreases the work function depending on the type of band bending. I'm shamelessly referring to Figure 1 of my paper :) Hope it helps!
-Peter
Article Surface Photovoltage-Induced Ultralow Work Function Material...
Peter Schindler Thank you for the answer. I am familiar with the concept of surface photovoltage and hence shift in the Fermi level in semiconductors with Fermi level pinning due to surface states (such as GaN). In the question, I referred to the n-type semiconductor whose Fermi level is not pinned, i.e., it is not affected by a large density of surface states, such as an n-type Si, which is often used as a substrate. It will be helpful if you can address this question.
Many thanks!
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