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KF5JRV > TECH 19.01.16 13:37l 65 Lines 4583 Bytes #999 (0) @ WW
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Subj: Diodes and LEDS
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Diodes and LEDs
The difference between conductors and insulators.
If you know a bit about electricity, you'll know that materials fall broadly into two categories. There are some that let
electricity flow through them fairly well, known as conductors, and others that barely let electricity flow at all, known as
insulators. Metals such as copper and gold are examples of good conductors, while plastics and wood are typical insulators.
What's the difference between a conductor and an insulator? Solids are joined together when their atoms link up. In something
like a plastic, the electrons in atoms are fully occupied binding atoms into molecules and holding the molecules together.
They're not free to move about and conduct electricity. But in a conductor the atoms are bound together in a different kind of
structure. In metals, for example, atoms form a crystalline structure (a bit like equal-sized marbles packed inside a box) and
some of their electrons remain free to move throughout the whole material, carrying electricity as they go.
Not everything falls so neatly into the two categories of conductor or insulator. Put a big enough voltage across any material
and it will become a conductor, whether it's normally an insulator or not. That's how lightning works. When a cloud moves
through the air picking up electric charge, it creates a massive voltage between itself and the ground. Eventually, the voltage
is so big that the air between the cloud and the ground (which is normally an insulator) suddenly "breaks down" and becomes
a conductor—and you get a massive zap of lightning as electricity flows through it.
Certain elements found in the middle of the periodic table (the orderly grouping of chemical elements) are normally
insulators, but we can turn them into conductors with a chemical process called doping. We call these materials Diodes and L
semiconductors and silicon and germanium are two of the best known examples. Silicon is normally an insulator, but if
you add a few atoms of the element antimony, you effectively sprinkle in some extra electrons and give it the power to
conduct electricity. Silicon altered in this way is called n-type (negative-type) because extra electrons can carry negative
electric charge through it.
In the same way, if you add atoms of boron, you effectively take away electrons from the silicon and leave
behind "holes" where electrons should be. This type of silicon is called p-type (positive type) because the holes can
move around and carry positive electric charge.
LED
An LED is a special type of diode (a type of electronic component that allows electricity to flow through in only one direction).
Diodes have been around for many decades, but LEDs are a more recent development.
They're tiny and relatively inexpensive.
They're easy to control electronically.
They last virtually forever. That makes them brilliant for traffic signals.
They make light electronically without getting hot and that means they save lots of energy.
Whom should we thank for this fantastic little invention? Nick Holonyak: he came up with the idea of the light-emitting diode
in 1962 while he was working for the General Electric Company.
LEDs are simply diodes that are designed to give off light. When a diode is forward-biased so that electrons and holes are
zipping back and forth across the junction, they're constantly combining and wiping one another out.
Sooner or later, after an electron moves from the n-type into the p-type silicon, it will combine with a hole and disappear.
That makes an atom complete and more stable and it gives off a little burst of energy (a kind of "sigh of relief") in
the form of a tiny "packet" or photon of light.
This list summarizes what happens:
1 N-type silicon has extra electrons.
2 P-type silicon has extra holes.
3 Battery connected across the p-n junction makes the diode forward biased, pushing electrons from the n-type to the p-type
and pushing holes in the opposite direction.
4 Electrons and holes cross the junction and combine.
5 Photons (particles of light) are given off as the electrons and holes recombine
LEDs are specifically designed so they make light of a certain wavelength and they're built into rounded plastic bulbs
to make this light brighter and more concentrated. Red LEDs produce light with a wavelength of about 630-660 nanometers
which happens to look red when we see it, while blue LEDs produce light with shorter wavelengths of about 430-500
nanometers, which we see as blue.
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