What is the difference between a diode and a led? I know a led is a diode and I know how they are used and so on but I'm not really sure why a regular is not also light emitting. Therefore, what is the key aspect a diode is missing so it can emit light?
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1$\begingroup$ See en.wikipedia.org/wiki/Direct_and_indirect_band_gaps. $\endgroup$– eranrechesCommented Feb 17, 2019 at 14:03
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1$\begingroup$ It purely depends upon the material used in making the diode. Below, the answers are available correctly. I just add (in simplified manner) that, if the normal diode is made of silicon, its transition of electrons between two energy bands (which is indirect) does not produce visible light. Whereas, in LED the material used like Gallium, indium has direct band gap. If electric field is applied here, the transition between the states, produce light called the process 'Electroluminescence'. $\endgroup$– Rajesh RCommented Aug 20 at 10:39
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$\begingroup$ thank you a lot, so it can be boiled down to direct and indirect band gaps? $\endgroup$– BenCommented Aug 22 at 8:10
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2 Answers
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In semiconductor physics, there is 2 types of band gap, a direct band gap and an indirect band gap.
Light-emitting and laser diodes are almost always made of direct band gap materials, and not indirect band gap ones like silicon.
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As others already pointed out, LEDs need to have a direct band gap, which means that the bottom of the conduction band and the top of the valence band must have the same momentum. This is necessary, because a photon at band gap energy carries almost no momentum, therefore the source and target state of the electron must have the same momentum for the process to take place. If the bandgap is indirect, i.e. the hole and the electron have different momenta, a recombination process can only take place with the assistance of a phonon, which carries away the difference in momentum.
