An electron existing in the conduction band is in a state called a meta-stable state and will ultimately become stable to a position of lower energy in the valence band. When this happens, the electron must move into a valence band state that is empty. Thus, when the electron becomes stable down into the valence band, as a result it occupies a hole. This process is defined as recombination.
Types of Recombination
For the bulk of a single-crystal semiconductor there are three main types of recombination which are:
- Radiative recombination;
- Auger recombination
- Shockley-Read-Hall recombination
1-Radiative Recombination (Band-to-Band)
The recombination mechanism that governs in direct bandgap semiconductors is radiative recombination. The light produced from an LED is the most clear illustration of the radiative recombination in a semiconductor. Space solar cells and concentrators are usually manufactured from direct bandgap materials (like GaAs) and radiative recombination rules. But, majority of terrestrial solar cells are manufactured from silicon, which is from the group of indirect bandgap semiconductor and radiative recombination is exceptionally low and ignored usually. The main properties of radiative recombination are:
An electron from the conduction band combines directly with a valence band hole and releases a photon
The photon emitted has a similar energy to the band gap and is subsequently just absorbed weakly so that it can leave the semiconductor material.
2-Recombination Through Defect Levels
Recombination through defects is also known as Shockley-Read-Hall (SRH) recombination, does not happen in absolutely pure, non defected material. SRH recombination is actually a two-step process. The two steps for SRH recombination are:
-An electron or a hole is stuck by an energy state in a prohibited region that is introduced by defects in the crystal lattice. These defects can be either introduced unintentionally or intentionally added to the material by doping the material for example
-When the hole or the electron jumps up to the identical energy state before the electron is re-emitted thermally into the conduction band, it then recombines.
The carrier movement rate into the energy level in the prohibited gap relies on the introduced energy level distance from either of the band edges. Thus, if the introduced energy is close to either of the band edges, recombination is less probable as the electron will probably to be re-emitted to the edge of the conduction band rather than be recombined with a hole moving into the same energy state from the valence band. Therefore, energy levels close to the mid-gap are really effective for recombination.
Auger Recombination encompasses three carriers. Rather than emitting the energy after an electron and a hole recombine as photon or as heat, the energy is provided to a third carrier, which is an electron in the conduction band. The electron thermalizes then back down to the edge of the conduction band.
Auger recombination is most significant at the high carrier concentrations triggered by high level injection under concentrated sunlight or by the heavy doping. In the silicon-based solar cells , lifetime and ultimate efficiency is limited by the Auger recombination. The further a material is heavily doped, the shorter the lifetime of Auger recombination.