From: Paul Doherty (pauld@exploratorium.edu)
Date: Tue Apr 03 2001 - 10:34:21 PDT
Message-Id: <l03110709b6efba0c862d@[192.174.2.173]> Date: Tue, 3 Apr 2001 10:34:21 -0700 From: Paul Doherty <pauld@exploratorium.edu> Subject: colored LEDs
Hi Eric
In LED's an electron in the N layer of a semiconductor diode combines with
a hole in the P layer. (picture an electron combining with a sodium ion to
make a neutral sodium atom.) In the combination within the LED, energy is
released a a single photon. The single photon has the energy released when
the electron fell into the hole.
By using different materials to make the N and P layers scientists can make
their own energy levels. Early red LEDs used Ga As P
modern blue ones use Ga N
High efficiency LEDs now also use Al In Ga P called "allengap"
The key idea is that LEDs convert one unit of electrical potential energy
into one unit of light energy, one photon. The electrical potential energy
is measured in electron volts, so one electron falling through 3 volts
makes a 3 eV photon which is blue.
White LEDs are actually cyan LED's with impurities added. The impurities
add energy levels other than the single one built in to the LED structure,
thus photons of many different wavelengths come out. (Look into a white LED
when it is off and you will see a yellow filter, this is used to remove
some of the blue light to make the white less blue.
Some green LEDs work better in this experiment than others.
You can also use infrared LEDs. You can see "infrared" using any digital
camera. Silicon CCD's are sensitive to infrared. Just look at your infrared
TV remote control with a digital camera and it will flash brightly in the
camera monitor.
Paul D
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
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