How does LED work?

How does LED work?


LED radiation phenomenon is associated with radiative recombination mechanism in a semiconductor. During the current flow through the p-​n junction charge carriers are injected thereto (holes from the p and the electrons from the n) and recombine there radially. The electroluminescent process in diodes can reach efficiency up to 50%. In the case of band-​band recombination, radiation energy is set to the width of bandgap.


There are two types of semiconductors:
Intrinsic semiconductor — is a pure semiconductor without any significant dopants. In such semiconductor there is only one possibility of transport of electric charge — energy must be supplied to electron(eg. by heating), greater than the bandgap energy, as it could find itself in the conduction band. In places abandoned by the electrons in the valence band, are being formed the so-​called holes, which are, easily explaining– positive charge carriers (electrons are negative charge carriers).
Extrinsic semiconductor – involving intentional introduction of other atoms into the crystal lattice, modifying its properties. There are two types of impurity atoms: donor and acceptor.
Acceptor dopants have three valence electrons that is one less than the crystal, in which those atoms are applied. In order to fill the fourth bond, electron is gathered out of the neighbouring atom, resulting in the excess hole. This type of semiconductor is named an acceptor (p type).


The donor dopant atoms have five valence electrons that is one more than the crystal in which those atoms are used. Four electrons create the conduction band with the crystal lattice, whereas the fifth one becames extra valence electron. This type of semiconductor is named a donor (type n).
In consequence of doping to a new band model, new energy levels and differentiation of light color are introduced.


Radiative recombination may occur in several ways:
–band to band recombination – electron from the conduction band directly combines with a hole in the valence band and releases a photon with an energy similar to the band gap
–recombination by a shallow acceptor levels – electron from the conduction band recombines with the hole from the acceptor level – produced photon with a lower energy is released
–recombination through the shallow donor levels – electron from the donor level combines with a hole from the valence band – photon is released
- donor-​acceptor recombination – electron from the donor level combines with a hole from the acceptor level
- recombination through deep levels – in such case, photon energy is much more lower than of energy gap.