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CN8-generation and recombination

CN8-generation and recombination - Excess carrier in...

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1 1 Excess carrier in Semiconductors, Generation and Recombination, and Quasi Fermi Levels 2 Outline Excess carriers, generation/injection and recombination/extraction process, and their imbalance Direct recombination rates Indirect recombination rates light absorption Common light generation/“luminescence” processes Steady-state carrier distributions Quasi ”-Fermi levels Summary
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2 3 Excess carriers in semiconductors Carriers can be added to a region by, o Electron-hole pair generation via Light/photon/optical absorption o Electron-hole pair generation via absorption and emission of phonons (lattice vibrations) o Combinations of the above o Electron-hole pair generation via carrier-carrier interactions o Electron and/or hole injection into the region of interest from another highlighted because they will be considered in this set of notes; the remaining will be considered as appropriate elsewhere 4 Carriers can be removed from a region by the physical reverses of the above processes, e.g., o Electron-hole pair recombination via Light/photon/optical absorption o Electron-hole pair recombination via absorption and emission of phonons (lattice vibrations) o Combinations of the above o Electron-hole pair recombination via carrier-carrier interactions o Electron and/or hole extraction from the region of interest into another
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3 5 In equilibrium each generation/injection processes is matched by the corresponding recombination/extraction process in detail ( detailed balance). When there are excess carriers, the system is inherently out of equilibrium … and one or more of these processes are necessarily out of balance , at least in “ detail .” 6 Carrier recombination, importance of: Most semiconductor devices are at least impacted by recombination of excess electrons and holes. Many, such as bipolar-junction transistor and lasers, depend on electron-hole recombination as a basic operating principle.
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4 7 Direct recombination process and rate/lifetime (Remember) electrons can recombined direct ly with holes (vacancies) via emission of light/photons as readily occurs in direct gap semiconductors … one photon of energy ħω E g per e-h pair . c v k k h h h e k k h h + 0 crystal momentum conservation, of which light has little, requires or, equivalently, Figure 3—5 (modified) Direct and indirect electron transitions in semiconductors: (a) direct transition with accompanying photon emission ; (b) indirect transition via a defect level. + + + + + + + + + + 8 Consider “ spontaneous recombination (the norm in all but lasers where recombination is also “induced,” TBD near the end of the semester) such that the time required for any given electron- hole pair combination to recombine is constant .
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