Wide Bandgap Extrinsic Photoconductive Switches Page: 79 of 245
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recombination center (electron trap) if the value of kn is greater (less) than unity. Similarly
for a hole-filled level at ED, kp is the ratio of the level electron capture rate (recombination) to
thermal emission rate of holes to the valence band.
k" - (353)
The values of ET and ED that cause kn and kp to be equal to one are called the electron and
hole demarcation levels, ED,1 and EDp, respectively. The quasi-Fermi and demarcation levels
are shown in Figure 3.15. The demarcation levels divide the band gap into regions of electron
trapping near the conduction band, hole trapping near the conduction band and the center
region where the traps act as recombination centers for both electrons and holes. If the optical
Conduction Band
Ec
Ec - EDn L or T'ap -
- - - - ----------Electron Quasi-Fermi Level
Electron Demarcation* "" " " """""""""""""""""""""""ee ee ee e ee ee ee ee
Line
Recombination Centers
Hole Demarcation * * * * * * * * * . . . * * e .. . ..... *.. ... 6
Line
Ev + Erip
Ev
Valence Band
Figure 3.15 Quasi-Fermi and demarcation levels in band gap of a semiconductor with
numerous traps
excitation is increased, the electron and hole quasi-Fermi levels and demarcation lines will
move closer to their respective band edges. The motion of the quasi-Fermi and demarcation
levels towards their respective band edges will change some electron and hole traps into77
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Sullivan, J S. Wide Bandgap Extrinsic Photoconductive Switches, thesis or dissertation, January 17, 2012; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc835750/m1/79/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.