Owing to the high refractive index of semiconductors, light incident on a planar semiconductor- air interface is totally internally reflected, if the angle of incidence is sufficiently large. Snell’s law gives the critical angle of total internal reflection. As a result of total internal reflection, light can be “trapped” inside the semiconductor. Light trapped in the semiconductor will eventually be reabsorbed, e. g. by the substrate, active region, cladding layer, or by a metallic contact.

If the light is absorbed by the substrate, the electron-hole pair will most likely recombine non-radiatively due to the inherently low efficiency of substrates. If the light is absorbed by the active region, the electron-hole pair may re-emit a photon or recombine non-radiatively. For active regions with internal quantum efficiencies of less than 100%, a reabsorption event by the active region reduces the efficiency of the LED.

The external quantum efficiency of an LED is the product of the internal quantum efficiency, nint, and the extraction efficiency, nextraction, i. e.

next = nint Extraction. (91)

The extraction efficiency thus plays an important role in increasing the power efficiency of

LEDs.