SRHRekombination

SRH recombination, named after Shockley, Read, and Hall, is a trap-assisted nonradiative recombination mechanism in semiconductors. It occurs when electrons and holes recombine via defect states, or traps, within the band gap. These traps are defects in the crystal lattice or impurities that introduce energy levels E_t between the conduction and valence bands. The recombination process typically involves capture of a free carrier by a trap, followed by capture of the opposite carrier and eventual recombination with energy released to the lattice as phonons.

The rate of SRH recombination depends on trap density, the energy level of the defect, and the capture probabilities for electrons and holes. A common practical expression for the recombination rate is R_SRH = (n p − n_i^2) / [τ_p (n + n_1) + τ_n (p + p_1)], where n and p are the electron and hole densities, n_i is the intrinsic carrier concentration, τ_n and τ_p are the electron and hole lifetimes related to the trap density and capture cross sections, and n_1 and p_1 are functions of the trap energy relative to the band edges. This framework can be extended to multiple traps but captures the essential dependence on defect density N_t, trap energy E_t, and capture cross sections σ_n and σ_p.

Impact and applications: SRH recombination can dominate in materials with many defects or poor crystalline quality, reducing minority-carrier lifetimes and device performance. In solar cells, it lowers open-circuit voltage and overall efficiency; in light-emitting devices it contributes to nonradiative losses. Mitigation strategies include material purification, defect passivation, surface passivation, and engineering of trap energies. SRH recombination is a fundamental consideration in device modeling and in interpreting carrier lifetime measurements across a wide range of semiconductors.