Under forward bias, the p-n junction is flooded with excess minority carriers: electrons pushed into the p-region and holes pushed into the n-region. Most of them recombine right at the junction. Each recombination event releases energy roughly equal to the bandgap E_g — but only some of that energy leaves as light.
λ (nm) = 1239.84 / E_g (eV) η_IQE = R_rad / (R_rad + R_nr)
- Bandgap energy — sets the photon's wavelength directly. Wider gap → shorter wavelength → bluer/UV light; narrower gap → longer wavelength → red/IR. This is why GaN LEDs are blue and GaAs LEDs are infrared, without changing anything else about the device.
- Defect density — mid-gap trap states (dislocations, impurities, surface states) capture a carrier and release its energy as lattice vibrations (phonons) instead of a photon: Shockley-Read-Hall non-radiative recombination. More traps means a dimmer LED for the same current.
- Forward bias — sets the carrier injection rate, i.e. how many electron-hole pairs arrive per second. It scales total recombination but does not, by itself, change the radiative/non-radiative split.
- Internal quantum efficiency — the fraction of recombination events that produce a photon. Real LEDs push this above 80% with epitaxial growth that minimizes defects; a cheap or damaged junction can fall well below that.
Real-world relevance: this is the electroluminescence mechanism behind every LED and OLED display pixel — the opposite process from a photovoltaic solar cell, which absorbs a photon to free a carrier pair instead of emitting one.