27-11-2012, 02:16 PM
THE LIGHT EMITTING DIODE
LED.ppt (Size: 952 KB / Downloads: 27)
When the electron falls down from conduction band and fills in a hole in valence band, there is an obvious loss of energy.
In order to achieve a reasonable efficiency for photon emission, the semiconductor must have a direct band gap.
What this means is that
as an electron goes from the bottom of the conduction band to the top of the valence band;
As we all know, whenever something changes
state, one must conserve not only energy, but also momentum.
In the case of an electron going from conduction band to the valence band in silicon, both of these things can only be conserved:
Phonons possess both energy and momentum.
Their creation upon the recombination of an electron and hole allows for complete conservation of both energy and momentum.
All of the energy which the electron gives up in going from the conduction band to the valence band (1.1 eV) ends up in phonons, which is another way of saying that the electron heats up the crystal.
Method of injection
We need putting a lot of e-’s where there are lots of holes.
So electron-hole recombination can occur.
Forward biasing a p-n junction will inject lots of e-’s from n-side, across the depletion region into the p-side where they will be combine with the high density of majority carriers.
Notice that:
Photon emission occurs whenever we have injected minority carriers recombining with the majority carriers.
If the e- diffusion length is greater than the hole diffusion length, the photon emitting region will be bigger on the p-side of the junction than that of the n-side.
Constructing a real LED may be best to consider a n++p structure.
It is usual to find the photon emitting volume occurs mostly on one side of the junction region.
This applies to LASER devices as well as LEDs.
MATERIALS FOR LEDS
The semiconductor bandgap energy defines the energy of the emitted photons in a LED.
To fabricate LEDs that can emit photons from the infrared to the ultraviolet parts of the e.m. spectrum, then we must consider several different material systems.
No single system can span this energy band at present, although the 3-5 nitrides come close.
Properties of InGaN
InGaN alloy has one composition at a time only.
This material will emit one wavelength only corresponding to this particular composition.
An InGaN LED would not emit white light (the whole of the visible spectrum at once) since its specific composition.
For a white light source we have to form a complicated multilayer device emitting lots of different wavelengths.
Orange-Yellow & Green LEDs
Orange (620 nm) and yellow (590 nm) LEDs are commercially made using the GaAsP system. However, as we have just seen above, the required band-gap energy for emission at these wavelengths means the GaAsP system will have an indirect gap.
The isoelectronic centre used in this instance is nitrogen, and the different wavelengths are achieved in these diodes by altering the phosphorus concentration.
The green LEDs (560 nm) are manufactured using the GaP system with nitrogen as the isoelectronic centre.
Blue LEDs
Blue LEDs are commercially available and are fabricated using silicon carbide (SiC). Devices are also made based on gallium nitride (GaN).
Unfortunately both of these materials systems have major drawbacks which render these devices inefficient.
The reason silicon carbide has a low efficiency as an LED material is that it has an indirect gap, and no ‘magic’ isoelectronic centre has been found to date.
Gallium Nitride (GaN)
Gallium nitride has the advantage of being a direct-gap semiconductor, but has the major disadvantage that bulk material cannot be made p-type.
GaN as grown, is naturally n++ .
Light emitting structures are made by producing an intrinsic GaN layer using heavy zinc doping. Light emission occurs when electrons are injected from an n+ GaN layer into the intrinsic Zn-doped region.