In forward biased PN-junction diode the current f1ow is occupied with the recombination of carriers. If the semiconductors are silicon and germanium the recombination is indirect. That is the recombination takes place by trapping the carriers in forbidden gape.

So the energy realizes by the recombination of such devices are in the form of' heat. If indirect recombination principle is used light can be generated instead of heat at the time of recombination. This defect is called injection elector luminescence and is used for LEDs. The photo energy emitted from the LED various from ultra-violet to infrared.

Normally varying the component percentage of semiconductor in alloy gallium arsenic, phosphate. The wavelength of the photon energy can be changed. Generally we can write the material for the LED is Ga,As .The different materials,which produces lights.

One of the first material used for LED is GaAs .This is a direct band gap material, it exhibits very high probability of direct transition of electron from conduction band to valence band. GaAs has Eg =1.44eV .This wavelength corresponds to infrared region. Thus GaAs made LED emits infrared ray. To achieve emission in visible range, one requires a higher band gap material than GaAs.

GaP and GaAsP are higher band gap materials. The value of GaP is 2.26eV with lamdag = 549mm. When group five elements are used as dopents, the radiate recombination's increase resulting in , improved efficiency. With nitrogen as doping the emission wave length is nearly 550nano meters ie. green radiation. At high room temperature the emission shifts to 590nm, i.e.: yellow radiation. Red emission is also possible from GaP by using(Zn&O) double doping .

GaAsP is useful LED material. It is a tertiary alloy. This material has a special feature in that it changes from being direct band gap material and indirect band gap material. Orange and yellow emission may be obtained from diodes of composition GaAsP respectively. Blue LEDs are of recently origin.The wide band gap material such as GaN(43.4eV) is the one of the most promising LED's for blue and green emission. Infrared LED's are also available.