why does a diode make light

LedCrazy

Newly Enlightened
Joined
Oct 29, 2003
Messages
3
Can someone please tell me (in laymans terms) how or why a diode produces light.
Scientific and nonscientific explanations welcome.

I've read about electrons changing energy levels but don't understand it.
I especially don't understand why a change in energy levels will produce light or absorb light (I probably have this
wrong but want to understand).
 

JJHitt

Enlightened
Joined
Dec 30, 2002
Messages
356
Location
Houston, TX
Lifted from Craig's site at
http://ledmuseum.home.att.net/reserved.htm#q0

An LED is basically a really fancy diode. Diodes only let current (electricity) to flow in one direction and not the other. LEDs are diodes too, but they have the unique "side effect" of producing light while electricity is flowing through them.

In the simplest terms, an LED is made with two different kinds of semiconductor material: one type that has too many free electrons roaming around inside, and another that doesn't have enough. When an electron from one material (the donor) gets pushed across a thin barrier and gets into tiny spaces in the other (the holes), a photon or particle of light is produced.
 

mattheww50

Flashlight Enthusiast
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Jun 24, 2003
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Location
SW Pennsylvania
What has been said is true as far as it goes, but there is some theory underneath that is very important. There is a potential called surface energy in the junction of a semiconductor device. This is the energy required to free an electron or charge carrier. Depending upon the choice of dopants (the deliberately introduced impurities, which is why LED's have such exotic chemical names associated with them), that energy ranges from about .4 volts for hot carrier devices, and can be massaged up to about .9 volts with a lot of effort and exotic compounds. So when electrons or charge carrier cross the junction and recombine, the energy that was expended to free the electron or charge carrier has to go somewhere. Mostly it is heat, but If you design it correctly, you can get some of that energy to appear as photons,
and it just so happens that a .4 electron volt has a wavelength that corresponds to Infra-red, and a .9v electron volt photon corresponds to Ultra violent. By picking the right dopant to create the desired band gap, you can produce differing LED colors. This difference also explains why higher voltages are needed produce light output from Blue LED's then from Infra-red. The extra voltage has to come frome somewhere. You probably cannot produce band gaps much higher than about 1 electron volt, since that is roughly bond energy for most semiconductor, ergo they are likely to become unstable if you could find a way to get the band gap up to the bond energy.
 
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