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Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
My understanding is that the difference between ordinary diodes and light emitting ones is the nature of the semiconductor- you need a direct gap to emit significant radiation.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks for the link.
Looks like the main difference between photons and phonons is the energy, exitons can interact with either.
"Excitons are the main mechanism for light emission in semiconductors at low temperature (when the characteristic thermal energy kT is less than the exciton binding energy), replacing the free electron-hole recombination at higher temperatures.
The existence of exciton states may be inferred from the absorption of light associated with their excitation. Typically, excitons are observed just below the band gap.
When excitons interact with photons a so-called polariton (also exciton-polariton) is formed. These excitons are sometimes referred to as dressed excitons.
Provided the interaction is attractive, an exciton can bind with other excitons to form a biexciton, analogous to a dihydrogen molecule. If a large density of excitons is created in a material, they can interact with one another to form an electron-hole liquid, a state observed in k-space indirect semiconductors."
And
"Normally, excitons in a semiconductor have a very short lifetime due to the close proximity of the electron and hole. However, by placing the electron and hole in spatially separated quantum wells with an insulating barrier layer in between so called 'spatially indirect' excitons can be created. In contrast to ordinary (spatially direct), these spatially indirect excitons can have large spatial separation between the electron and hole, and thus possess a much longer lifetime."
It seems you are more likely to have electrons remain in the conduction band with indirect transfer. Both seem to have a certain amount of overlap. The main differences appear to be with the time factors involved.
It's in fig 3a of the paper I cited and it flattens out below about a milliamp. So the number of photons per electron does not suddenly drop to zero at some small current.
You should look at the 25C curve in fig 3a. It might also flatten out, but at a considerably lower level. Fig 2 indeed shows light output below V=hw/q.
Ash Small wrote:
I suppose the reason I made the analogy with a BJT is mainly regarding the current flow reported above, where photons are visible down to ~2V with a blue LED, below which point current can still be detected flowing, but no photons can be detected.
A typical LED in the usual operating range can have a 50% quantum efficiency, but that drops significantly for low voltages. There are very few thermally excited electrons then, which attain the energy required for emission of light. Most electrons pushed to higher energy by the applied voltage will lose their energy by scattering on imperfections of the lattice or contaminants. That doesn't mean that there is no light output. It will be just difficult to measure against the background of thermal radiation. The LED used for the paper emits in the far infrared, the emission energy low, so thermal effects are much larger compared to blue LEDs. In addition, their LED was heated for this experiment.
Phonons and photons both have energies and wavelength. But e.g. electrons and protons do also. Phonons are lattice vibrations, photons are "vibrations" of the electromagnetic field.
BC: We're looking at the same diagram. The lower black data points seem to indicate a levelling off. From the data alone it's not completely convincing. There is a theoretical reason for levelling off, though. At lower currents, i.e. lower voltages, the fraction of electrons with enough energy for a light emitting transition is smaller. Correspondingly more electrons will lose their energy by non optical decay. At some voltage the ratio won't change much more. I'd expect levelling off at voltages (currents correspond to these) scaling with k*T.
Ash:
That's a really interesting graph. What exactly are the units for the Y axis, I'm not familiar with that nomenclature?
The Y axis shows the fraction of the number of photons emitted relative to the number of electrons conducted through the LED.
EDIT: I'm still pondering the wavelength of a banana
In QM there is a wavelength lambda associated with every object:
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Ash Small wrote ...
That's a really interesting graph. What exactly are the units for the Y axis, I'm not familiar with that nomenclature?
EDIT: I'm still pondering the wavelength of a banana
There are no units, what is plotted is a number. It is (as I pointed out earlier) the ratio of the number of photons per unit time, divided by the number of electrons per unit time.
I said a bunch of bananas. this is what a bunch of bananas looks like
and, for clarity, this is what a hand of bananas looks like
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Was 14V LED question
