Generation and Recombination of Carriers

By Mallesham Devasane|Updated : July 1st, 2016

Recombination: A process whereby electrons and holes are annihilated or destroyed.

Energy Bands in Recombination Processes

  • Band-to-Band Recombination: Also referred to as direct thermal recombination, is the direct annihilation of a conduction band electron and a valence band hole. The electron falls from an allowed conduction band state into a vacant valence band state. This process is typically radiative with the excess energy released during the process going into the production of a photon (light).
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  • R-G Center Recombination: A defect often causes an energy state in the midgap region of the bandgap that can act as traps for carriers. In this process, a conduction band electron gets trapped at this defect and energy state and then a valence band hole is trapped and recombines with the electron or vice versa. This process is typically nonradiative.
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  • Recombination via Shallow Levels: Donor and acceptor sites can also act as intermediaries in the recombination process similar to the R-G Center recombination with the exception that the recombination is quite often radiative.
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  • Recombination Involving Excitons: Conduction band electrons and valence band holes can form a slightly lower energy state by becoming bound to each other in a hydrogen-like arrangement. The electron and hole can then recombine and result in a photon that has energy slightly less than the bandgap.
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  • Auger Recombination: A nonradioactive recombination event where a conduction band electron losses energy to another conduction band electron which gains energy to result in one electron recombining with a hole in the valence band and one high energy electron which rapidly losses energy by phonon emission and relaxes back down to the conduction band.

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Generation: A process whereby electrons and holes are created.

The recombination processes can be reversed resulting in generation processes.

Band-to-Band generation occurs when an electron from the valence band is excited by light or heat to the conduction band.

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Generation of an electron and hole by an R-G center intermediary can be done in a couple of ways including and electron from the valence band being excited to the trap state and then to the conduction band creating a hole in the valence band and electron in the conduction band.

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Also, an electron in the trap state can be excited to the conduction band while the hole is excited to the valence band.

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Another common carrier generation is carrier generation via impact ionization where a high energy electron losses energy to produce an electron-hole pair.

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