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Carrier generation describes processes by which electrons gain energy and move from the valence band to the conduction band, producing two mobile carriers; while recombination describes processes by which a conduction band electron loses energy and re-occupies the energy state of an electron hole in the valence band.
There are several mechanisms by which minority carriers can recombine, each of which subtract from the carrier lifetime. The main mechanisms that play a role in modern devices are band-to-band recombination and stimulated emission, which are forms of radiative recombination, and Shockley-Read-Hall (SRH), Auger, Langevin, and surface recombination, which are forms of non-radiative recombination.
When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier recombination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity.
Excess carriers in the material cause it to become more conductive, and thus the number of excess carriers can be measured over time by measuring the material conductivity. Conductivity can be measured through non-contact methods, such as through microwave reflectance, or inductive or capacitive coupling . [ 3 ]
Auger recombination is a similar Auger effect which occurs in semiconductors. An electron and electron hole (electron-hole pair) can recombine giving up their energy to an electron in the conduction band, increasing its energy. The reverse effect is known as impact ionization. The Auger effect can impact biological molecules such as DNA.
where the js are the current densities of electrons (e) and holes (p), the μs the charge carrier mobilities, E is the electric field, n and p the number densities of charge carriers, the Ds are diffusion coefficients, and x is position. The first term of the equations is the drift current, and the second term is the diffusion current.
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is the rate of the incident photons that are absorbed in the photogeneration, is the rate of the density of free carriers in the generation layer reduced by recombination, is the injection rate. Assuming that the charges cross the interface will not return, the photoinjection efficiency Υ {\displaystyle \Upsilon } can be defined as [ 10 ]