Light emitting diode (LED) is a component that converts the electrical signal into a corresponding light that be injected into the fiber. Basically, the light emitter is a key element in any fiber optic system. Essentially LED is a PN junction diode.
Heterojunctions
A (heterojunction) is an interface between two adjoining single-crystal semiconductors devices with a different bandgap.
Heterojunctions are of two types, Antisotype ( p-n ) or, Isotype (n-n or p-p)
Double Hetero-junctions (DH)
In order to achieve efficient confinement of emitted radiation double hetero-junctions are used in LED structures. In double hetero-junction, the crosshatched regions represent the energy levels of free charge recombination occurs only inactive InGaAsP layer.
A hetero-junction is a junction formed by dissimilar semiconductors. Double heterojunction ( DH) is formed by two different semiconductors on each side of the active region. Figure shows double heterojunction (DH) light emitter.
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Double heterojunction emitter ( DH )
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The two materials have different refractive indices and different bandgap energies. This structure can change in bandgap energies create the potential barrier for both holes and electrons and also some free charges can recombine only in narrowband, well defined active layer side.
A double hetero-junction structure will confine both holes and electrons are narrow active layer. So in DH junction under forwarding bias, it will be a large number of carriers injected into active regions where they are efficiently confined. One of the most advantages of LED serves to narrow the output beam.
Advantages of LED structure :
- Carrier recombination occurs in a small region
- light emission occurs in an optical waveguide, which serves to narrow the output beam.