What is recombination?
In semiconductors, there are two types of charge carriers: electrons and holes. Electron-hole pairs (EHPs) are generated thermally and photonically (with light). Since they are oppositely charged, they tend to attract each other. Holes are essentially a lack of an electron, so when an electron meets a hole, it combines with it. The electron and hole essentially cancel out. This cancellation is called recombination.
In some applications, recombination is desired. Light emitting diodes (LEDs) work because of recombination. Recombination is induced in LEDs to produce light. In solar cells, great efforts are made to avoid recombination. Recombination in solar cells reduces the amount of power it produces. Power is produced because carriers move through a circuit, but recombination prevents them from moving.
Recombination produces a depletion region when a p-type and n-type semiconductor are placed together. Holes from the p side move to the n side to recombine, as electrons from the n side move to the p side. At the junction, an excess of holes are found on the n side and an excess of electrons are found on the p side. This induces a built-in voltage and thus an electric field. Away from the depletion region, the n and p sides are electrically neutral. The depletion region itself is made from recombination and is used in electronics. Transistors use these depletion regions to make a switch that’s used in logic for computing.
How does recombination happen?
There are three types of recombination: Radiative, Shockley-Read-Hall, and Auger (pronounced “oh-jay”).
Radiative, or band-to-band, recombination occurs when an electron and hole directly recombine. A photon is produced with the same energy as the bandgap. This is the most common type of recombination seen in direct bandgap semiconductors. While it does occur in indirect semiconductors, it is negligible. Direct bandgap semiconductors have the peak of the valence band (VB) directly below the nadir of the conduction band (CB), thus facilitating radiative recombination. LEDs are designed to produce light of different colors (thus energies) by using materials with different bandgaps. A larger bandgap would produce a higher energy photon and correspond to bluer light, while a shorter bandgap would produce redder light.
Shockley-Read-Hall (SRH) recombination
Every material has some defects. These defects, commonly impurity atoms or grain boundaries, make excellent trap spots for electrons. These traps provide lower energy states for the electrons. Naturally, they drop to these trap states. Recombination is facilitated because the trap states bring the electrons closer to the VB edge where they can recombine with a hole. These states can also absorb differences in momentum between carriers via phonons. Thanks to this, indirect semiconductors are most affected by SRH recombination. SRH recombination is also dominant in direct semiconductors with very low carrier densities.
Here, an electron and hole recombine and thus release energy. However, the energy released during recombination is transferred to another particle. The additional energy brings this particle to a higher energy state. The particle eventually relaxes by releasing energy as heat, or phonons, in a process called thermalization. This happens in non-equilibrium conditions like heating and illumination. Because it is so significant in non-equilibrium conditions, solar cell lifetime is defined partly by Auger recombination lifetime.