Date: Mon Oct 6 17:08:17 2008

Author: Adam Beehler

Subject: LED's in LN2

Post:

Tappers,

I finally have an instructor that is interested in showing a
demonstration I put together after listening to this group. I of course
put it together awhile ago and now do not remember exactly how it
works. Every time I give the instructor an explanation, he comes at me
with something else. Well, I have no more ammo to dish out. Anyway,
the demo is lighting an LED (let's say it is green). Then, without
changing anything, I dip the LED into liquid nitrogen and the LED now
glows yellow. Remove it from the liquid nitrogen and as it slowly warms
back up it glows green again. One can increase the voltage across the
LED while it is cold and not blow it. Is this sounding familiar to
folks? I have temporarily posted a link to a video I took of it here:

http://www.physics.utah.edu/~beehler/uofu/pnJxnLEDLN2.mpg

So.....what I would like to know is how it really works. Does the
temperature really change the band gap energy and thus the photons
emitted? Or is there some type of diffusion layer that changes its size
and thus effectiviely changes the band gap? Or is it just colder so it
emits light of lower energy? Does anything physically change inside the
semiconductor material? Thanks for any added insight!

Adam Beehler


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