LED's Flashcards
Carrier recombination
When the electron falls down from conduction
band and fills in a hole in valence band, there is an
obvious loss of energy.
indirect band-gap material,
-Para o eletrão ir da banda de condução para a bamda de valencia tem de mudar o momento
-Tem no entanto de ocorrer conservação do momento
-Para ir da banda de condução para a de valencia geram-se então fonões
-Os fonões tem energia como momento, permitindo conservação do momento
Direct band-gap semiconductors
-Não há variação de momento(O que é bom pq os fotões não tem momento associado)
-O excesso de energia da recombinação resulta em calor ou um fotão de luz.
-Esta transição conserva energia e dá luz quando um eletrao e um buraco recombinam
Resumindo
Direct Bandgap:
–Base of the conduction band is matched to the max height of the valence band
–Recombination through the emission of a photon
*Indirect Bandgap
–direct recombination would require a momentum change
–Recombination centers are required to recombine CB to VB bands
–The result is a phonon emission (lattice vibration) that propagates across the lattice
Recombination centers
-In addition to the band-to-band transition, radiative recombination could be happened via recombination centers generated by impurities.
Some indirect gap materials can have a reasonable efficiency if they are doped with certain impurities (isoelectronic center)
-The isoelectronic center is an efficient radiative recombination center and can largely enhance the
probability of radiative processes in an indirect gap material
Isoelectronic centers
-Isoelectronic means that the centre being introduced has the same number of valance electrons as the element it is replacing.
-It doesn’t act as a dopant.
-Isoelectronic centres provide a ‘stepping stone’ for electrons in so that transitions can occur that are radiatively efficient.
-Because the effective transition is occurring between the isoelectronic centre and VB edge, the photon that is emitted has a lower energy than the band-gap energy (avoids re-absorption)
Non-radiative recombination
- Defeitos relacionados com níveis de energia podem surgir de impurezas químicas ou de defeitos estruturais
-In a LED the non-radiative recombination is not desirable, but it is purposely increased in p-n diodes to increase the speed.
-The defected-related non-radiative recombination is also called Shockley-Read-Hall recombination. Such recombination is very important at the surfaces of
devices, since there is usually a high density of defects.
LEDs basic operation mechanism
-LEDs are pn junctions usually made from direct bandgap semiconductors
-Direct electron hole pair (EHP) recombination results in emission of a photon
-Photon energy is approximately equal to the bandgap energy, Eg
-Application of a forward bias drops the depletion region allowing more electrons into the p side of the device and increasing the probability of recombination in the depletion region
-The recombination zone is called the active region
-Light emission from EHP recombination as a result of minority carrier injection as shown here is called injection electroluminescence
-The statistical nature of this process requires that the p side be sufficiently narrow to prevent
reabsorption of the emitted photons
Injection electroluminescence
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.
LEDs materials(White)
There are two primary ways of producing high intensity white-light using LEDs:
-One is to use individual LEDs that emit three primary colors (red, green, and blue), and then mix
all the colors to produce white light.
-the other is to use a phosphor material to convert monochromatic light from a blue or UV LED
to broad-spectrum white light, much in the same way a fluorescent light bulb works.
Materiais fosforescentes
-Phosphors are stable materials and can have quantum efficiencies of close to 100%. Dyes
also can have quantum efficiencies of close to 100%.
-Dyes can be encapsulated in epoxy or in optically transparent polymers. However, organic
dyes have finite lifetime.
LEDs structure
LEDS are typically formed by epitaxially growing doped semiconductors layers on suitable
substrate
-film and the substrate have mismatched lattice sizes then the
lattice strain on the LED leads to crystalline defects that cause indirect recombination of EHPs
and a loss of electroluminescence (photon emission). Thus the substrate is usually the same
material as the epitaxially layers
-To insure that recombination occurs on the p side, the n side is very heavily doped. Photons emitted toward the n side become absorbed or reflected back at the substrate interface
White LEDs
-Wavelength converter materials include phosphors, semiconductors and dyes.
-The parameters of interest are absorption wavelength, emission wavelength and quantum efficiency
-Common wavelength converters are phosphors, which consist of an inorganic host material doped with an optically active element.
-The optically active dopant is a rare earth element, oxide or another compound.
Photon loss
-absorption in the material (making the junction close to the
emitting surface)
-absorption in the substrate area (using a transparent substrate)
-total internal reflection at the surface (suppressed by using a dielectric encapsulation dome)
-transmission losses at the interface
(minimized by using a dielectric encapsulation dome)
-reflection at the top M/S interface
(minimized by using as small an area as possible of top metal contact)
-absorption in the top contact metal
(minimized by using as small an area as possible of top metal contact)
LEDs efficiency
Do mais eficiente ao menos:
-band-to-band recombination in direct gap material,
– recombination via isoelectronic centres,
– recombination via impurity (not isoelectronic) centres,
– band-to-band recombination in indirect-gap materials.
