Explain to me how LEDs work

Having been a LED crazed person since the dark ages, I thought that I knew how LEDs work. Last week, when my father inlaw asked me what is it about LED flashlights that are so interesting to the point of obsession. I replied back, its all about the LED. He said, well tell me how LEDs work. I hemmed and hawed but what I said did not make sense to him. Right there I realized that I really don't know how LEDs work.

I suspect that many of you gentle CPF readers are also in the same boat. I would appreciate it if someone could either explain to me the inner workings of LEDs or point me in the right source. What I am looking for is not a journal article on LEDs but a dummys guide to LEDs.

Thanks (and sorry if this topic has been addressed before).

Regards.

I hope you don't consider this a "journal article", but to me this page gives good simple explanations.

http://electronics.howstuffworks.com/led.htm

It's actually more difficult to explain than I thought as well. I mean, to make it simple (when it isn't that simple a concept for me to begin with).

This is my take on it:

LED's produce light as high energy electrons enter a semiconductor material doped to be deficient in low energy electrons (full of "holes" for low energy electrons) and transition across a gap in the semiconductor material into the "holes". They are forced into the holes by the current flow and in doing so are forced to release energy in order to occupy the holes. The energy is released in the form of photons of light. The frequency of the light depends on the size of the gap the electrons must cross.

Is it really that more difficult to explain how metals give off light when a current is passed through them?

Or, it's sometimes easier to say "magic" /ubbthreads/images/graemlins/icon15.gif

This is one of those questions that's easy to ask but hard to answer.

Here's how I explain it to psych & sociology majors:

An LED is made up of two layers of silicon. Think of them as two areas of real estate.
One layer has most of its electrons at a high energy level. The other layer has most of its electrons at a low energy level.
When you apply an electric current to the two layers you push some of the high energy electrons into the low energy real estate. When an electron goes from the high energy real estate to the low energy real estate it loses its extra energy.
That extra energy isn't destroyed. It zooms off in the form of a photon. Photon = Light
Q.E.D. (This is usually when they ask "what is Q.E.D.?")
Good luck explaining it to people without having their eyes glaze over..........

No I can't. And I don't care how they work. I do care that they more than likely will never have to be replaced and are more resistant to damage than an incandescent bulb.
I have learned that Led's do weaken (dim) with usage.

I have carried the title "engineer" for over 25 years but I prefer Size15s' answer better: "Magic." /ubbthreads/images/graemlins/smile.gif

Larry

I think we covered that about three-quarters of the way through my graduate-level semiconductor physics course. Which was more years ago than I care to remember.

MicroE's description sounds pretty good. I remember that it has something to do with the band-gap energy of electrons transitioning between hyperfine levels in the orbitals of the crystaline lattice. Which produces photons with the energy of the bandgap energy.

I think "Magic" is the best description.

I think that my father inlaw would produce quite a chuckle If I was to simply say, trust me, it's just "magic." /ubbthreads/images/graemlins/crackup.gif /ubbthreads/images/graemlins/crackup.gif

Regards.

[ QUOTE ]
MicroE said:
This is one of those questions that's easy to ask but hard to answer.

Here's how I explain it to psych & sociology majors:

An LED is made up of two layers of silicon. Think of them as two areas of real estate.
One layer has most of its electrons at a high energy level. The other layer has most of its electrons at a low energy level.
When you apply an electric current to the two layers you push some of the high energy electrons into the low energy real estate. When an electron goes from the high energy real estate to the low energy real estate it loses its extra energy.
That extra energy isn't destroyed. It zooms off in the form of a photon. Photon = Light
Q.E.D. (This is usually when they ask "what is Q.E.D.?")
Good luck explaining it to people without having their eyes glaze over..........

[/ QUOTE ]

Minor point though - LEDs are not made from Silicon, and they are not made from 2 layers. Probably still ok for soc. majors.

OK, I will take a crack at it. This is not entirely correct, but good enough for most laymen.

You can simplistically think of atoms as being similar to the planets - The sun is the nucleus, and the planets are the electrons.

When energy (such as electrons) pass through the atom, the atom absorbs the energy by moving the electrons from one orbit to another. The atom releases the energy when the electrons drop back to their original orbit. When this happens, light is emitted.

In the case of LEDs, the individual atoms are replaced by crystal layers called "quantum wells". The color of the light can be altered by modifying the crystal layer composition and thickness. As a general guide, red and amber LEDs are made from AlInGaP, and the rest are made from InGaN alloys.

OK, not a college physics answer but it will get most people by.

Something to do with di-lithium crystals, I believe. /ubbthreads/images/graemlins/smile.gif

Seriously, very interesting information. I can finally form a semi coherent answer when someone asks me how is that different from a Mag lite?

I read the howstuffworks explanation and I could follow some of it. I don't think that I could fully explain it like that though. I was hoping that someone would give me the Readers Digest Condensed version. For those who are physics-challanged like me, I again say thanks in advance.

From what I have read above, the explanation has something to do with di-lithium crystals and magic . . . .

Regards.

Here is a college primer with a very simple applet that explains it using a simple graphic representation.

FSU primer

OK, going one step further in ease of explanation. Your father probably remembers "Tube Radios", and while he might not understand what a transistor is, he has the general idea.

Tell him that the LED is like a transistor, and an incan light is like a Tube from a tube radio. He will get the picture.

[ QUOTE ]
HarryN said:
Minor point though - LEDs are not made from Silicon, and they are not made from 2 layers. Probably still ok for soc. majors.

[/ QUOTE ]

Harry---You are correct, but I generally ignore this detail. I talk about the LED in their TV remote control because it is a very familiar cozy LED.

It's just easier to talk to people about silicon since they know that integrated circuits are generally made from silicon and it makes sense to them that LED's would be made from the same stuff.

Trying to explain high-bandgap III-V semiconductors just complicates the situation and gets questions like
"Arsenic, they're made from DEADLY arsenic?"

I have also given up correcting people every time they say "silicone" chips. Most people prefer their science in "sound bite" format. Details be d@mned.

For the record, silicon LEDs can only produce infra-red (invisible) wavelengths due to their small bandgap.

Icebreak, thanks for the lead to the FSU Primer - it is the best explanatiion so far although I am still unclear what happens when voltage is applied to the substrate layers to produce light.

Regards.

I can't explain it in layman's terms either. My explanation will get into such things as donors, holes, quantum bandgaps, and other horse puckey like that.
The only "simple" eplanation I can offer is right here. And even it is probably incorrect and/or has glaring ommissions. /ubbthreads/images/graemlins/jpshakehead.gif

If someone asked me how an incandescent bulb works, I wouldn't dive down
into the physics of how the wire is heated by the current, how some of
that energy is converted into light, etc. I'd probably just say "the little
wire inside gets hot enough to glow". If they want more, they're probably
ready for more.

What's the equivelant answer for an LED? How 'bout "It's made of a special
type of material that converts some elecricity directly into light without
having to get as hot as a normal bulb".

Not the best, but it's at the same level of abstraction as the incandescent
bulb explanation.

Okay, I'll take a crack at it. If I commit any howlers, please note it's been 17 years since I've taken any college-level physics /ubbthreads/images/graemlins/smile.gif

Okay, I do think it's possible to explain this to a layman who has at least some basic physics knowledge. Here's how I'd start explaining it:

Electrons form a cloud around an atom. The electrons are in "orbits", and those orbits correspond to different energy levels. An electron with a relatively higher energy level is said to be in a higher-energy orbit than an electron with a lower energy level. When an electron in a high-energy orbit drops down to a lower-energy orbit, it has to shed the extra energy, and it does so by releasing a photon (light). What an LED does is create the conditions where high-energy electrons drop to low-energy electrons, and shed the extra energy as light.

That's probably enough to make most people happy, but what if they ask for more detail (perhaps, like me, they've had physics 101! /ubbthreads/images/graemlins/smile.gif Then you can launch into the more detailed explanation:

First, you take a material that has some electrons missing from it. Don't worry about how we create a material that has missing electrons, just trust me, they're missing. We call the spots where the electrons should be, but are missing, "holes". Since this material has missing electrons, it is positively charged.

Then we take a material that has extra electrons in it (see previous note on how we created this material). Since this material has extra electrons, it's negatively charged.

As a side note, the two materials I just described above are based on the exact same stuff, but different types of impurities are added (this process is called "doping") to get the material to end up with extra electrons or holes.

Okay, now, we take the two materials above and we put them together. When we apply a voltage in the correct direction across this material, the extra electrons from the negatively-charged side move towards the junction where the materials meet, and the holes from the positively-charged side also move towards the juncture. At the juncture, the holes and electrons meet, the electrons fall into the holes (recall a "hole" is a spot where an electron is missing). When an electron falls into a hole, it drops to a lower energy level, and as we know, gives off the extra energy as a photon.

So, now the question is: if electrons are moving one way (towards the center of the material), and holes are moving the other (also towards the center, but from the other side), how does a current get generated? It takes a bit of thought, but if you think about it, electrons moving one way is exactly the same thing as holes moving the other way. Think about a bubble rising to the surface of a glass of water. You can think about the bubble (the hole) moving upwards through the liquid. Or, you can think about the bubble being stationary, but the liquid flowing downwards around the bubble. You can model the process either way, but the results are the same: the hole ends up at the top, the liquid at the bottom.

That's exactly what we're seeing with the LED. If you want to think of current as "electrons moving from left to right", then what you have is (say) electrons moving from the leftmost lower-voltage direction to the right towards the LED's juncture, and you have holes moving from the higher-voltage side moving from the rightmost higher-voltage direction towards the LED's juncture. But since we already said that "holes moving from right to left" means exactly the same thing as "electrons moving from left to right", that's how electrons get all the way around the circuit.

Joe

I think EmmissionLine is on the right track. We all accept that a hot filament produces light, yet very few care about the physics behind it.