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VK3ABK > LEDS 31.07.03 19:23l 51 Lines 2319 Bytes #999 (0) @ WW
BID : 62922_VK3KAY
Read: GUEST OE7FMI
Subj: RE: What are modern LEDs made from.
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Sent: 030730/1324Z @:VK3KAY.#WEV.VIC.AUS.OC #:62922 [Wendouree] $:62922_VK3KAY
From: VK3ABK@VK3KAY.#WEV.VIC.AUS.OC
To : LEDS@WW
Hello technophiles.
Nick, ZL3TPN has asked about LEDs and Lasers, a subject which he suggests
"is probably a big one". He's right: and a knowledge of solid state
electronics, and some familiarity with quantum mechanics would be useful.
But a rather simplified comment in this bulletin may be of some interest,
and satisfy Nick and other readers. Digging back into memory, and a quick
look at a text on Optoelectronics, I have this to offer.
A Light Emitting Diode emits radiation in several wavelengths (colours).
What we see is the result of electrons falling from one energy level to a
lower one. As energy cannot be 'lost', only converted, we see photons
emitted from a P N junction which forms the LED. This junction is made up
of Gallium with other substance added. Gallium-Phosphide gives red light
or green with further doping of the Gallium (N) electrode. A combination of
Gallium-Arsenide-Phosphide gives orange/red, and Gallium-Arsenide gives us
Infra Red. Incedentally, I have used diode combinations such as these in a
research project requiring detection of Infra Red scintillations.
"How are LEDs made". Specifically, I don't know; but I suppose that vapour
deposition of the P layer on a base of doped Gallium would be used, similar
to the manufacturer of transistors and other solid state devices.
Nick asked about lasers. Normal LEDs are not lasers, as their radiation is
random; not 'coherent' or in phase. A diode laser (or any other laser) must
be 'pumped' to raise the electron energy levels, before a fall to a lower
level causes visible radiation. This radiation is then both coherent and
has low divergence, giving an intense, narrow beam.
"Do LEDs wear out"? I suspect we will see bulletins of anecdotal evidence
supporting the yes or no vote. But I doubt that the electrons or photons
decay, although impurities in the basic elements may cause a long term
reduction in emission, or a shift in wavelength. The easiest way to shorten
the life of a LED is to reverse the polarity of the applied voltage.
Take particular note of the permitted reverse voltage. It's very low!
For more details I could delve into 'Optoelectronic Devices and Principles'.
I hope this is of help to Nick, and also of general interest.
73. Dick. VK3ABK
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