I read that an LED (primarily emits light) can also be used as a photodiode (primarily detects light). Is this true in reverse? I mean, can a photodiode also emit light?
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12\$\begingroup\$ Give it enough power and yes, it will (briefly) emit light... and smoke... and noise ;) \$\endgroup\$– MajenkoOct 14, 2014 at 12:49
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2 Answers
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Yes it can, and does. Any recombination of the carriers (electron and holes) which happens all the time will emit light with an energy level corresponding to the bandgap, 1.12 eV (minus just a little bit) ~ 1 um for Silicon. Although, it must be noted that this is a very inefficient process. It is not used directly because of that but is used in some diagnostic tools.
answered Oct 14, 2014 at 13:12
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\$\begingroup\$ Do I only need to connect it in reverse? \$\endgroup\$– KozuchOct 14, 2014 at 13:15
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1\$\begingroup\$ Oops, a typical PIN silicon photodiode will not emit light. It's an indirect band gap material. Now a GaAs photodiode may... I've never used one and don't know for sure. \$\endgroup\$ Oct 14, 2014 at 13:50
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\$\begingroup\$ @GeorgeHerold That is flat out wrong, it IS a very inefficient process, it needs to involve k-space (phonons) to emit but it will emit nevertheless. Surface effects, traps and defects all will emphasize the process. If you were right then some very key diagnostic equipment wouldn't work. \$\endgroup\$ Oct 14, 2014 at 14:36
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\$\begingroup\$ @Kozuch the recombination rate will increase if forward biased. Keep in mind that since the emission is essentially at energy of the bandgap, you can't use Si to detect this. Probably InGaAs would be your best choice. \$\endgroup\$ Oct 14, 2014 at 14:38
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1\$\begingroup\$ @placeholder, I pulled Pankove ("optical processes in Semi's") off the shelf. He lists the intrinsic radiative recombination time for some semi's. Si is 4.6 hours... typical numbers are in the microsecond range. GaAs is not listed. \$\endgroup\$ Oct 14, 2014 at 15:32
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A direct bandgap photodiode can certainly emit light, and the amount of light can be modulated by loading it differently.
Shine light above the bandgap energy on a III-V junction and you will make carriers. If you have left the leads unconnected rather than reverse-biased it, then those carriers may spend enough time near each other so that a significant fraction will recombine with each other, rather than at non-radiative sites such as defects and dopants.
In effect, photodiodes will "fluoresce" at the bandgap energy. Short it or reverse bias it, and you can collect more of those e-h pairs and the glow (during illumination) will be reduced.
It doesn't really matter if you create e-h pairs electrically or photonically, once they are there, a fraction will undergo radiative recombination.
See for example the answer(s) to Do III-V based photovoltaics “glow” (photo-luminesce) when illuminated but not loaded?
