Recombination, Generation, and Non-Equilibrium Carrier Dynamics
Non-Equilibrium Carrier Populations
Quasi-Fermi levels
Under illumination or injection, electrons and holes may be described by separate quasi-Fermi levels and . Their separation is a direct measure of non-equilibrium and is especially important in photovoltaics, LEDs, and high-injection devices.
Diagnostic quantity
The splitting often determines the maximum attainable open-circuit voltage in illuminated devices.
Recombination Mechanisms
Radiative, Auger, and trap-assisted processes
Radiative recombination produces photons and is strong in direct-gap materials. Shockley-Read-Hall recombination proceeds through defect states in the gap and is often dominant in imperfect materials. Auger recombination is a three-carrier process important at high carrier densities, especially in narrow-gap semiconductors and lasers.
Major recombination channels
| Mechanism | Typical dependence | Common importance |
|---|---|---|
| Radiative | Direct-gap optoelectronics | |
| SRH | Trap-assisted, lifetime-limited | Defective or compensated materials |
| Auger | Often scales strongly with carrier density | High injection, narrow-gap systems |
Which recombination process is explicitly defect-mediated?
SRH recombination occurs through trap states in the band gap.
Correct answer: Shockley-Read-Hall recombination
What does the splitting of quasi-Fermi levels indicate?
In equilibrium there is a single Fermi level; splitting signals non-equilibrium carrier populations.
Correct answer: A departure from equilibrium caused by carrier injection, generation, or optical pumping.
Continuity and Lifetime
Carrier continuity
The continuity equation for electrons is , with a corresponding equation for holes. In small-signal analysis, lifetimes and diffusion lengths arise from linearization around steady state.
The carrier lifetime is most directly connected to:
Lifetime quantifies the characteristic recombination time scale for excess carriers.
Correct answer: How long carriers survive before recombining under a given perturbation
Name one reason SRH recombination can dominate in practical materials.
Trap levels enhance recombination even when radiative pathways are weak.
Correct answer: Because defects or impurities introduce gap states that act as efficient recombination centers.