Carrier Actions

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Recombination
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Recombination

Recombination is part of a process to restore equilibrium to a semiconductor that has been perturbed, or disturbed out of equilibrium. Perturbations can be in the form of an applied electric field, a change in temperature or exposure to light. Recombination occurs when there is an excess of carriers and they are destroyed, by recombining. Remember carriers are in the form of a free electron and the hole it leaves behind, or electrons or holes brought in from dopants. When they are destroyed, a negatively charged electron is attracted to a positively charged hole, and as they get together, their charges are canceled and the electron is part of a bond once again. There are three types of recombination that we cover in this course. These are band-to-band recombination, R-G center recombination and Auger recombination.

Band-to-band recombination is the annihilation of a conduction band electron and a valence band hole simply by having the attraction of the negative charge of the electron and the positive charge of the hole cancel each other out as they move in the semiconductor lattice. The energy released during the process usually produces a photon and emits light.

In R-G center recombination there are special locations within the semiconductor which are lattice defects or special impurity atoms that introduce allowed electronic levels near the center of the band gap. These levels are important because they will not be mistaken for donor or acceptor levels. R-G center recombination involves a two step process. A carrier and electron, for example, strays into the vicinity of an R-G center, an electronic level near mid-gap, and is trapped. Later comes a hole and the electron is attracted to the hole and loses more energy so it goes into the valence band annihilating the hole along with itself, this is called indirect recombination. This method releases thermal energy or produces lattice vibrations.

Auger recombination is a type of band-to-band recombination that occurs when two carriers collide. The collision transfers the energy released from the recombining carrier to the surviving carrier. In other words, one carrier loses energy and the other gains it. The one that loses it is recombined, and the one that gains it goes to a higher energy level. Eventually, this highly energized carrier "thermalizes" - loses energy in small steps through heat producing collisions with the semiconductor lattice, until it eventually recombines or gains energy once more.

The three recombination mechanisms are similar in the fact that they all annihilate an electron for each hole, and that they all produce some kind of energy, whether it be heat or light, in the process. They are all different in the fact that band-to-band recombination only uses the fact that the carriers of opposite charge happen to be in the same vicinity, no outside interference is needed. R-G center recombination uses defects in the crystal lattice or special impurities that are added to the semiconductor material to add energy levels to where the carriers can move up or down, closer to their opposite charge so the attraction can destroy them. Auger recombination uses collisions between electrons to transfer energy from one electron to another, sending one to a higher energy and another to a lower one where it is attracted to a hole and they are both annihilated.


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