21-11-2012, 01:01 PM
LED Characteristics
LED3.ppt (Size: 699.5 KB / Downloads: 146)
LED Construction – Aim – 100% light emitting efficiency
Important consideration - radiative recombination must take place from the side of the junction nearest to the surface to reduce reabsorption.
Carrier from n must be injected into the p-side efficiently.
Consider the fraction of the total diode current that is carried by electrons being injected into the p-side of the junction (e)
LED Characteristic
The energy of an emitted photon = to the size of the band gap
BUT this is a simplified statement.
The energy of an emitted photon from LED is distributed appropriately according to the energy distribution of electrons on the conduction band and holes in the valance band.
You need to know the distribution of electrons and holes in the CB and VB respectively.
The quantum efficiency
Internal quantum efficiency can of some LED approaches 100% but the external efficiencies are much lower. This is due to reabsorption and TIR.
III-V materials have small critical angles therefore the radiation emitted suffers from TIR
How to solve TIR problem
GaAs-air interface, the C = 16o which means that much of the light suffers TIR.
To solve the problem we could:
Shape the surface of the semiconductor into a dome or hemisphere so that light rays strike the surface angles < C therefore does not experience TIR. But expensive and not practical to shape p-n junction with dome-like structure.
Encapsulation of the semiconductor junction within a dome-shaped transparent plastic medium (an epoxy) that has higher refractive index than air.
Homo- and Hetro-Junction
Homojunction = a p-n junction made out of two differently doped semiconductors that are of the same material (i.e having the same band gap).
Heterojunction = junction formed between two different band gaps semiconductors.
Heterostructure device = semiconductor device structure that has junctions between different bandgap materials.