Any electron which exists in the conduction band is in a meta-stable state and will eventually fall back to a lower energy position in the valance band. It must move back into an empty valence band state and consequently, when the electron falls back down into the valence band, it also effectively removes a hole. This process is called recombination. There are three basic types of recombination in the bulk of a single-crystal semiconductor. These are:

• Radiative recombination;
• Auger recombination; and
• Shockley-Read-Hall recombination.

These are described in the animation and text below.

Radiative (Band-to-Band) Recombination

Radiative recombination is the recombination mechanism that dominates in direct bandgap semiconductors. The light produced from a light emitting diode (LED) is the most obvious example of radiative recombination in a semiconductor device. Concentrator and space solar cells cells are typically made from direct bandgap materials (GaAs etc) and radiative recombination dominates. However, most terrestrial solar cells are made from silicon, which is an indirect bandgap semiconductor and radiative recombination is extremely low and usually neglected. The key characteristics of radiative recombination are:

• In radiative recombination, an electron directly combines with a hole in the conduction band and releases a photon; and
• The emitted photon has an energy similar to the band gap and is therefore only weakly absorbed such that it can exit the piece of semiconductor.

Recombination Through Defect Levels

Recombination through defects, also called Shockley-Read-Hall or SRH recombination, does not occur in perfectly pure, undefected material. SRH recombination is a two-step process. The two steps involved in SRH recombination are:

• An electron (or hole) is trapped by an energy state in the forbidden region which is introduced through defects in the crystal lattice. These defects can either be unintentionally introduced or can have been deliberately added to the material, for example in doping the material; and
• If a hole (or an electron) moves up to the same energy state before the electron is thermally re-emitted into the conduction band, then it recombines.

The rate at which a carrier moves into the energy level in the forbidden gap depends on the distance of the introduced energy level from either of the band edges. Therefore, if an energy is introduced close to either band edge, recombination is less likely as the electron is likely to be re-emitted to the conduction band edge rather than recombine with a hole which moves into the same energy state from the valence band. For this reason, energy levels near mid-gap are very effective for recombination.

Auger Recombination

An Auger Recombination involves three carriers. An electron and a hole recombine, but rather than emitting the energy as heat or as a photon, the energy is given to a third carrier, an electron in the conduction band. This electron then thermalizes back down to the conduction band edge.

Auger recombination is most important in heavily doped or heavily excited material.