LEDs are semiconductor devices that emit light via the recombination of electrons and holes within the active region (a sequence of specially designed layers) of the device. Most of these recombination events are termed “radiative” because they result in the production of a photon of light.
The wavelength of the emitted light is determined by the bandgap of the active region - a wider bandgap results in higher energy emission, which equates to a shorter wavelength.
In turn, the bandgap is determined by the chemical composition and sequence of the semiconductor layers within the active region.
**********
External Quantum Efficiency (EQE)
The ratio of the number of photons emitted from the LED to the number of electrons passing through the device - in other words, how efficiently the device coverts electrons to photons and allows them to escape.
EQE = [Injection efficiency] x [Internal quantum efficiency] x [Extraction efficiency]
- Injection Efficiency
In order that they can undergo electron-hole recombination to produce photons, the electrons passing through the device have to be injected into the active region. Injection efficiency is the proportion of electrons passing through the device that are injected into the active region
Internal Quantum Efficiency (IQE - also termed Radiative Efficiency)
Not all electron-hole recombinations are radiative. IQE is the proportion of all electron-hole recombinations in the active region that are radiative, producing photons.
Extraction Efficiency (also termed Optical Efficiency)
Once the photons are produced within the semiconductor device, they have to escape from the crystal in order to produce a light-emitting effect. Extraction efficiency is the proportion of photons generated in the active region that escape from the device.
**********
Wall-Plug Efficiency (also termed Radiant Efficiency)
Wall-plug efficiency is the ratio of the radiant flux (i.e the total radiometric optical output power of the device, measured in watts) and the electrical input power i.e the efficiency of converting electrical to optical power.
Wall-Plug Efficiency = [EQE] x [Feeding efficiency]
- Feeding Efficiency
Each electron-hole pair acquires a certain amount of energy from the power source when the LED is operating. Feeding efficiency is the ratio of the mean energy of the photons emitted and the total energy that an electron-hole pair acquires from the power source.
