[NA] How do LEDs work?

Let'sgoflying!

Let'sgoflying!

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Dave Taylor
http://en.wikipedia.org/wiki/Led

I read it but think I need it dumbed down. I can see a coil of metal glowing when electrons are run through it but here; "When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon."

Help. What is happening, please?! How do LEDs make light? No tech words greater than grade 3 level allowed.
 
In addition to what you said, I believe the distance the electron travels to meet the "hole" determines the color of light emitted.

Let'sgoflying! said:
http://en.wikipedia.org/wiki/Led

I read it but think I need it dumbed down. I can see a coil of metal glowing when electrons are run through it but here; "When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon."

Help. What is happening, please?! How do LEDs make light? No tech words greater than grade 3 level allowed.
Click to expand...
 
I'll try not to oversimplify, although I'm sure the physicists in the crowd will cringe at this.

When an electron moves from one orbital energy state (remember Freshman Chemistry?)around a nucleus to another it requires additional energy to move to a higher orbital energy state and releases energy (a photon) when it moves to a lower energy state. The LED takes advantage of that phenomenom to produce light. The materials used in LEDs provide just the right shift in electron energy states to produce the desired colored or white light.

For a long time pure white light from an LED was hard to produce with the technology at hand. Now it has become so common and so energy efficient that the incandescent lamp may become a museum piece in the not too distant future.
 
Let'sgoflying! said:
No tech words greater than grade 3 level allowed.
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I'm seeing a lot of words greater than a grade 3 level or maybe people were much more advanced in grade three than I was. :confused:

I think it's PFM. You hit the switch and the light comes on. What else do you need to know? :)
 
Let'sgoflying! said:
http://en.wikipedia.org/wiki/Led

I read it but think I need it dumbed down. I can see a coil of metal glowing when electrons are run through it but here; "When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon."

Help. What is happening, please?! How do LEDs make light? No tech words greater than grade 3 level allowed.
Click to expand...
I had an entire semester devoted to developing exactly how LEDs and regualr diodes worked. We even made an LED, I got an A, but I cannot remember anything about how they actually function other than it is something about an energy state change that throws off a photon. I do remember talking about how blue LEDs would be really hard to make, but I cannot remember why.

I do recall thinking that I am glad I was not going to be working in semiconductor development and instead was going into radio and communications.
 
Aw, heck, this is easy.

Put it in a wall outlet and it'll light bright for a second. :yikes: It's just sparks between two conductors... :P

OK, somebody had to say it....

Dave, asking for a simple explaination of this is sorta like asking for very simple explaination of how a neon or flourescent bulb works. Not that easy. The stuff that you referenced is the kind of stuff that I learned in science and engineering classes in college. I'm not sure I can explain it in grade-school terms.
 
Ok. Here's the analogy.

Think of an atom as our solar system. The planets are electrons. The sun is the nucleus. If we add energy to the solar sytem, Earth might move to the orbit of Mars. When energy is removed from the system, electrons will fall to a lower orbit - Mars moving to Earths orbit, or Earth to Venus'. If the electron known as Mars falls to a lower orbit, or shell, it has to give off energy to do so. (Energy is neither created nor detroyed, it just changes form) This energy given off as a photon.

There are multiple electrons in each orbit, and they move orbits when subjected to, or have energy taken from the atom.

Is that a better explanation?
 
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wsuffa said:
Aw, heck, this is easy.

Put it in a wall outlet and it'll light bright for a second. :yikes: It's just sparks between two conductors... :P

OK, somebody had to say it....

Dave, asking for a simple explaination of this is sorta like asking for very simple explaination of how a neon or flourescent bulb works. Not that easy. The stuff that you referenced is the kind of stuff that I learned in science and engineering classes in college. I'm not sure I can explain it in grade-school terms.
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You could do the 'magic smoke' explination. That is to say that semiconductors are filled with magic smoke, if oyu let the smoke out the semiconductor no longer works.
 
EdFred said:
Is that a better explanation?
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That's much more third-grade, but I already understood that part about the electrons going to a smaller orbit and giving up energy in the form of light. The part I don't understand is why flipping the switch and starting the flow of electricity causes this.
 
Everskyward said:
That's much more third-grade, but I already understood that part about the electrons going to a smaller orbit and giving up energy in the form of light. The part I don't understand is why flipping the switch and starting the flow of electricity causes this.
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Electricity (energy) goes into the atom on one side of the diode, increasing the energy, and moving electrons outwards in their shells. The atoms move across the diode, and lose the energy given to them by the electricity. That energy has to be given up somehow. In incandescent bulbs it it heat and the glowing wire. In LEDs the electrons drop inwards and give off photons. The electrons of atoms always want to find the lowest orbit possible inside the atom. And with the laws of conservation they can't move orbits without taking on or giving up energy.
 
EdFred said:
Electricity (energy) goes into the atom on one side of the diode, increasing the energy, and moving electrons outwards in their shells. The atoms move across the diode, and lose the energy given to them by the electricity. That energy has to be given up somehow. In incandescent bulbs it it heat and the glowing wire. In LEDs the electrons drop inwards and give off photons. The electrons of atoms always want to find the lowest orbit possible inside the atom. And with the laws of conservation they can't move orbits without taking on or giving up energy.
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OK thanks. That is a good third grade explanation!
 
Just to complicate it: The material I have read on Quantum Physics indicates that there is suspicion among physicists that (A) electrons do not "orbit" the nucleus; that (B) electrons are not particles, but small packets of energy of some sort; that (C) light is either both a particle and a wave, or maybe neither. We DO know that light is part of the electromagnetic spectrum.

I'm no physicist but I think that as we get farther into the atom we're in for some real surprises. That new accelerator in Europe might result in more questions raised than answers found.

Dan
 
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If you want some real brain melting stuff, read up on Bose-Einstein condensates and supercooled molecules.
 
smigaldi said:
Did you see that Nova special on trying to reach absolute zero?? It is available on line if you did not and it was really great.
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Thanks for the heads' up! http://www.pbs.org/wgbh/nova/zero/program.html

Dave, that same sentence on "This involves a drop from the conduction band to a lower orbital" lost me as well, and I think it's because they had not defined "the conduction band" in the material leading up to that sentence.

Here's an article about it: http://en.wikipedia.org/wiki/Conduction_band
 
docmirror said:
I covered this stuff in post 6. You may want to edit this sentence above. Particle Physics doesn't lend itself to 3rd grade comprehension level. Sorry......
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Did Mari understand how diodes work with your post? No.
Did she understand with mine? Yes.

Don't be upset you can't explain it so the layman can understand.
 
EdFred said:
Did Mari understand how diodes work with your post? No.
Did she understand with mine? Yes.
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Exactly. When someone asks for a simple explanation they are not looking for words like P-type and N-type and 2N^2 and valence. OK, so I remember that valence is a word but I couldn't tell you what it means. My last exposure to chemistry and physics was in school decades ago and I wasn't real fond of the subjects at that time, as I recall. If Ed isn't absolutely, technically correct it doesn't matter to me because at least I know a little more than I did this morning...
 
Thanks! I think I am good on the dropping to a lower energy level, and release of photons as a result thing....
How about the actual physical set-up? I think in the gas-bulbs (neon/fluorescent) the gas is the medium that is energized, then releases photons when their molecules return to a lower energy state.
In the LED, is the medium simply the solid component of the LED itself??
Also, there is no spark (electrons jumping across a gap) with this, correct?
Finally, do you need DC current for an LED?
 
Let'sgoflying! said:
Thanks! I think I am good on the dropping to a lower energy level, and release of photons as a result thing....
How about the actual physical set-up? I think in the gas-bulbs (neon/fluorescent) the gas is the medium that is energized, then releases photons when their molecules return to a lower energy state.
In the LED, is the medium simply the solid component of the LED itself??
Also, there is no spark (electrons jumping across a gap) with this, correct?
Finally, do you need DC current for an LED?
Click to expand...

Short answers:

An LED is a junction diode and the light is emitted by the junction.

There is no "spark" just electron/hole current.

LEDs only emit when the voltage is applied with the correct polarity. If you apply AC to a LED it would only emit during half the cycle.

Long answers:
A LED is a semiconductor which means that there's some type of crystal substrate that's doped (infused with impurities) to make a "junction" (boundary between portions with different doping). The two sides of the junction are doped with specific impurities that have extra electrons on one (N) side and a deficiency of electrons on the other (p) side. This is the basic structure of any solid state diode. LED substrates are materials whose valence electrons have potential energies that differ by the level needed to produce photon's of the desired wavelength. The substrate also has to be transparent to light at that wavelength.

To make the LED emit light, a current is passed through the junction in the forward direction (positive on the P side). This current provides the external energy to raise the valence electrons to their next higher state and that energy is released as a photon when the energized electron reverts to it's natural stable level.

The amount of light (number of photons per second) is roughtly proportional to the current but that current is limited because this current also causes heating of the device as the electrons bounce around inside. There is also a point where the quantity of impure atoms available for "energizing" decreases enough to reduce the efficiency of the LED. The voltage drop across the LED junction varies with temperature and the current vs voltage curve is very steep near the potential required to emit light so in any LED circuit there must be some method of limiting current. This can be as simple as a series resistor or as complex as a constant current source (current regulator circuit).

LED output is inherently fairly monochromatic (one color) and there are two ways that white LEDs are made. One is to have the LED emit blue or near UV light which strikes a phosphor that absorbs the short wavelength light and emits a broad spectrum. The other approach is to combine two or three LED chips in one package with each producing a different color which when combined appear to be white. Some pigments will appear to be very different colors when illuminated with the two different types of white LEDs.

Some LEDs consist of two different color chips that are internally connected back to back (one diode is reversed in polarity from the other). The colors are usually red and green. With this type if you apply DC current in one direction you get green and if you reverse the current you get red. If you apply AC both chips will emit their color which combines to yellow as sensed by our eyes.



And although you didn't ask about laser diodes they are
very much related to LEDs:

A solid state laser is essentially a LED with mirrored sufaces on each end (facet) of the device. This causes most of the photons emitted to bounce back and forth through the junction many times and the photons trigger "sympathetic" releases of additional photons (that's the "stimulated" part of Light Amplification by Stimulated Emission). The mirrors are spaced an integral number of wavelengths of the emitted light apart so the wavefronts stay alighned between passes. The mirrors aren't perfect reflectors so some of the light eventually leaks out of one or both ends. Most laser diodes also have a photodiode (kind of a reverse LED) which produces a current proportional to the emitted light. This is used in a control circuit to regulate the output. Without this a laser diode would likely destroy itself by generating so many photons that the surface of the device would fracture.
 
Let'sgoflying! said:
In the LED, is the medium simply the solid component of the LED itself??
Click to expand...

If I understand what I think you're asking, yes. Note that most LED's have a lens above them, the lit part itself can be very small. (Doesn't have to be, though.)

Also, there is no spark (electrons jumping across a gap) with this, correct?
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Right. No spark, very very little heat. Safer, from a fire perspective.

Finally, do you need DC current for an LED?
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Technically, no - But an LED, like any diode, will only illuminate when voltage is applied in one direction. If you were to plug an LED into your wall - Well, OK, maybe through a transformer, call it 60Hz AC at 6 Volts peak to peak - It would blink 60 times per second, and be illuminated just slightly under half of the time.

Visualize the sine wave of the electricity, going from +3V to -3V, starting at the origin (0 V, and in an upward direction). There is a "diode drop" across a diode, which IIRC is around 0.7V for a normal diode, and 1.0V for an LED. Until the power is at +1 V, the diode will not be lit. From the time the wave reaches +1V, through the peak at 3V, and back down to +1V, it will be lit. From the time it drops below +1, through 0, to -3, back up through 0 to +1, the LED will not be lit. At 60Hz, you'll see it flickering. In fact, we had some cheap LED Xmas lights this year that did that. Ugh.

So, for lighting purposes, DC is better. Incandescents don't have much flicker because they're heated up pretty much the whole time, with the peaks of the sine wave "recharging" the heat for when the 0 crossings happen. Fluorescents generally don't flicker, especially on a newer bulb and a fixture with a modern ballast, but they can. LED's on DC won't flicker at all.
 
Let'sgoflying! said:
Got it, I am ready for the test. (and will be happy with a high 'C'!)
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I'll continue to believe "wiffle dust, magic, and mirrors" for most things. That way, I can stay out of the archives of my mind. A truly dark and scary place to be.
And I'm not likely to buy a LED lightbulb to replace an incandescent any time soon. Have you seen those prices?
 
I'm curious about one more thing on this topic... if an LED emits photons (light) at a specific wavelength that our eye perceives as a specific color, why do some LED's have a color plastic dome (i.e., "red")? Wouldn't our eye see red light anyway?

(Note: the LED article referenced above showed that the dome helps to concentrate the light out the tip of the bulb--got that. But why is the bulb tinted a specific color? Isn't the light emitted by the diode already that color?)
 
oops I just discovered I'd missed an important part of this.....
Why does it have to be a diode that is used?
Why not some multidirectional electronic component?
Is there something about the electricity going only one way that makes this work?
 
Let'sgoflying! said:
oops I just discovered I'd missed an important part of this.....
Why does it have to be a diode that is used?
Why not some multidirectional electronic component?
Is there something about the electricity going only one way that makes this work?
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Think of it as a cliff in the Grand Canyon (N-type material). You're the atom... you step off the edge (into the canyon/hole [P-type material]). You let out a scream (photon).

It doesn't work on flat ground--you have to have someplace to "fall into".
 
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silver-eagle said:
And I'm not likely to buy a LED lightbulb to replace an incandescent any time soon. Have you seen those prices?
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But those 1-watt LED flashlights! Not the early ones that had 10 or 12 or whatever number of white LEDs in them, but the newer ones with the Luxeon-type single LED. As an aircraft mechanic I've always been on the lookout for a better flashlight and have tried numerous brands and styles, but my favorite now is a one-watt LED version that sold three to a package, compete with all the AAA Duracells to power them, for $17 at Costco. Those AAAs run for a LONG time, and because the light doesn't go yellow as the batteries weaken I have to remember to replace them once I notice that it doesn't seem as bright as it used to. These lights make the Maglite look sick, believe me. Super-bright, slightly bluish light that makes inspections of dark places so much easier. Now I'm watching for an economical unit that takes AAs instead of AAAs since they're so much cheaper and will run twice as long.

Dan
 
TangoWhiskey said:
Think of it as a cliff in the Grand Canyon (N-type material). You're the atom... you step off the edge (into the canyon/hole [P-type material]). You let out a scream (photon).

It doesn't work on flat ground--you have to have someplace to "fall into".
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That's one way to put it.

Bottom line is there has to be a junction (Ted's cliff) and a diode is the simplest semiconductor geometry that has one junction.
 
TangoWhiskey said:
I'm curious about one more thing on this topic... if an LED emits photons (light) at a specific wavelength that our eye perceives as a specific color, why do some LED's have a color plastic dome (i.e., "red")? Wouldn't our eye see red light anyway?

(Note: the LED article referenced above showed that the dome helps to concentrate the light out the tip of the bulb--got that. But why is the bulb tinted a specific color? Isn't the light emitted by the diode already that color?)
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Generally correct, Troy, But I think that they make the cases in the desired color both for marketing (it's supposed to be red, so it looks red), and for practical (in a production or use environment, how do you tell a red from a green w/o either marking, an additional production step and markings would be necessarily tiny, or differentiated color lens).
 
SCCutler said:
Generally correct, Troy, But I think that they make the cases in the desired color both for marketing (it's supposed to be red, so it looks red), and for practical (in a production or use environment, how do you tell a red from a green w/o either marking, an additional production step and markings would be necessarily tiny, or differentiated color lens).
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Our nav lights are LED which appear clear when the power is off. However, the manufacturer has put a strip of red tape around the base of the red one and green tape around the green one.
 
Everskyward said:
Our nav lights are LED which appear clear when the power is off. However, the manufacturer has put a strip of red tape around the base of the red one and green tape around the green one.
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You can specify colored or clear lenses on LEDs. With the high out LEDs that are available it does seem that clear lenses are more prevalent.
 
smigaldi said:
You could do the 'magic smoke' explination. That is to say that semiconductors are filled with magic smoke, if oyu let the smoke out the semiconductor no longer works.
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Not limited to semiconductors. :D

The ultimate irony is who I work for with the grade I got in my solid state electronics class in college. :blush: There's probably a reason they haven't let me near a fab in the 14 years I've worked here. :P
 
smigaldi said:
You could do the 'magic smoke' explination. That is to say that semiconductors are filled with magic smoke, if oyu let the smoke out the semiconductor no longer works.
Click to expand...

Or to keep it LED related, another common Electrical Engineering student phrase:

"ALL diodes are light emitting.........once."

--Carlos V.
 
Unbeliever said:
Or to keep it LED related, another common Electrical Engineering student phrase:

"ALL diodes are light emitting.........once."

--Carlos V.
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Resistors, too. :D

That's what's fun about engineering. Get your answer wrong in other majors and all you get is a red check mark. Get the answer wrong in engineering, and you get fireworks. :goofy:

I've only had it happen twice. Once, the resistor began to flame - It looked like a candle for a few seconds. The other time, as soon as we turned the circuit on, one of the resistors went "POP!" and completely vaporized, leaving the two thin metal leads sticking up out of the breadboard with nothing between them.

Lab partner: "What just happened?"
Me: (poring over the notebook) "Well, see the error in this calculation here? We just put three hundred watts through a quarter-watt resistor." :D
 
That works in troubleshooting, too- if you keep blowing fuses, sometimes you just put in a bigger fuse so the defective component will fry, already.
 
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