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4hv.org :: Forums :: General Science and Electronics
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Uhh, i just built a light emitting antidiode...

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Adam Munich
Thu Jul 08 2010, 12:12AM Print View
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Location: SF Bay, CA
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It's a SiC crystal about .7cm2, jbwelded to a brass plate. It has point contacts.




Now here's the strange thing; it conducts electricity, and emits light... BOTH WAYS. I really can't explain this...
Pin on left negative



Pin on right negative




Also, it is by no means efficient. It draws 3A at 12v to emit a measly estimated .5mcd. Everything else turns to heat, VERY fast.

Can anyone explain this?
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Mattski
Thu Jul 08 2010, 01:25AM
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What you have is two schottky diodes (metal semiconductor junction) in antiseries. Each metal-semiconductor contact forms a diode, which can point in either direction depending on whether the work function of your contact is greater or smaller than that of the SiC. In either case, when sufficient bias is applied, one will enter reverse breakdown, and the other will then be forward biased and conducting current, and then able to emit light.

Given which side lights up, I think your diodes are forward biased with the semiconductor positive wrt the metal, which would mean metal has a lower work function than your SiC.




Cool work, where did you pick up the SiC?
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Adam Munich
Thu Jul 08 2010, 01:40AM
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That makes sense. I picked up the crystal at a rock shop a few years ago. It's the smallest one, but it was the most efficient at making light.

Another interesting thing, the wavelength gets longer as it gets warm. At room temp it's yellow, but after a few seconds when the temperature raises to about 500*F, it's orange. Maybe i'll spray some canned air or butane on it to cool it and see if it turns green.
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Mattski
Thu Jul 08 2010, 04:59AM
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That makes sense, as most semiconductors have a bandgap that decreases with temperature. I wasn't able to find a definitive reference on SiC, but it has been measured in straight silicon:

I found one paper which vaguely mentioned a 0.03eV increase in the bandgap from room temp to 2K. It might be subscription locked for you, but here it is:
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IntraWinding
Thu Jul 08 2010, 12:17PM
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Great stuff Grenadier! (and no chance of dosing yourself with X-Rays this time )

Matski: Would Grenadier get better results with a pair of different metal contacts such that one has a higher and one a lower work function than SiC?
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Adam Munich
Thu Jul 08 2010, 06:58PM
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I just used safety pins because they are springy and hard. I'm not sure any other metal than steel that pins are made of.
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IntraWinding
Thu Jul 08 2010, 07:17PM
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Grenadier wrote ...

I just used safety pins because they are springy and hard. I'm not sure any other metal than steel that pins are made of.


Good choice - I was just wondering what's next! Maybe a bit of copper wire, or a pin from an IC would work differently.

Is the crystal transparent, or is the light only emitted from its surface?

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Adam Munich
Thu Jul 08 2010, 07:20PM
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It's black as can be. Most of the light comes from the surface, but when power is applied so the "half' glows (not the circle), it looks like there is light coming from inside too. Somehow it becomes slightly translucent.
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Mattski
Thu Jul 08 2010, 11:08PM
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IntraWinding wrote ...

Matski: Would Grenadier get better results with a pair of different metal contacts such that one has a higher and one a lower work function than SiC?

He could, because then he could make it so that both schottky contacts are forward biased at the same time. I'm not sure what metals would be good for that though.

Another approach is to make only one contact a diode contact. This is a problem faced in any semiconductor process, making so called "Ohmic Contacts" which behave like a resistor rather than a diode. Ordinarily a good approach is to heavily dope the contact site, but that's tricky unless you try to rig up something like Jeri's doping/oxidation oven. I think you might get good results by just making a firm contact over a large surface area.

I came across a dissertation which is relevant:
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Conundrum
Fri Jul 09 2010, 10:08PM
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heh... i have seen three colours of light from one SiC sample before..

I usually sit the crystal in low melt BiSnPb alloy as it holds the crystal securely.

to connect to the top emitting contact i found that conducting thread works fairly well, so do pogo pins.

interestingly many non-emitting (at low voltage) areas emit light at 50V 1mA or so.

-A
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Adam Munich
Fri Jul 09 2010, 10:12PM
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The only "low melt alloy" i have is mercury, and i suppose that wouldn't make too good of a mount...
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Mattski
Fri Jul 09 2010, 10:23PM
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You could try solder, it melts at a comparatively low temperature, though probably not as low as Conundrum's BiSnPb.
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Conundrum
Fri Jul 09 2010, 11:14PM
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if anyone has a use, pm me and i can send them a few pieces from the crystal I used along with some mounting alloy.



regards, -A
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Adam Munich
Fri Jul 09 2010, 11:39PM
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Problem is, This LED reaches a temperature of well over solder's melting temperature, it uses 24W to make that .5mcd.
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803
Sat Jul 10 2010, 04:08PM
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G,

could you write a basic plan so I could play with it? I want to see the effects of higher voltage, cold and heat, and stuff.
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Adam Munich
Sat Jul 10 2010, 11:30PM
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Location: SF Bay, CA
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Plan? I just took two saftey pins at 12v and poked them at my SIC chunk. This was the best working crystal, so i broke it off and jbwelded it to a brass piece. Then i soldered the pins in a way that they always touch the SIC in the right way.

I'll play around with temp and volts when i get back home on monday.
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803
Sun Jul 11 2010, 12:00AM
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where did you buy the crystal? Are they a special type?
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Adam Munich
Sun Jul 11 2010, 02:44AM
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A local rock shop. Just search on ebay for the biggest SiC crystals you can find.
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