Re: pinhole RE: LED's

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Apr 04 2001 - 10:52:02 PDT 

Message-Id: <l03110719b6f10d8d4594@[192.174.2.173]>
Date: Wed, 4 Apr 2001 10:52:02 -0700
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole RE: LED's

Hi Eric

The color chromaticity diagram in the book Seeing the light shows that
light with a wavelength as long as 780 nm is visible red.

Diodes themselves have an exponential increase in current with voltage,
this can be catastrophic so they are usually placed in series with an
external resistor making them ohmic. Thus indeed you are right, the higher
voltage produces more current, and each electron in that flow of current
produces a photon. More photons means brighter.

A pure PN junction semiconductor has only 1 energy in its band gap.
So produces one pure color.
Add impurity atoms however and you can insert energy levels into the band
gap. These energy levels allow electrons to drop through a lower potential
energy producing a different wavelength than the bandgap. Add enough
impurities to get a wide enough range of wavelengths and you get white
light.

And the equations for the energy of the photon is hf = qV the energy of the
electron.

where h is planck's constant, f is the frequency of light, q is the charge
on the electron and V is the voltage drop across the bandgap.

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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