Diodes and LEDs
Last updated: March 16, 2009.
Move over bulbs: there are better ways
to make light now!
There are those compact
fluorescent lamps, for example—the ones that
save you energy and money. But, even better, there are LEDs
(light-emitting diodes) that are just as bright as bulbs, last
virtually forever, and use hardly any energy at all. 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.
Let's take a look at how these things work.
Photo: LEDs are smaller than small incandescent bulbs
(used in flashlights) and use a fraction as much energy. They are particularly suitable for use in instrument
panels, which have to be lit up for hours at a time.
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 a very
orderly climbing frame) and some of their electrons remain free to
move throughout the whole material, carrying electricity as they go.
How semiconductors work
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
(which is normally an insulator) between the cloud and the ground
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
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 (shown here as black blobs) 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 (shown here
as white blobs) can move
around and carry positive electric charge.
How a junction diode works
Interesting things happen when you start putting p-type and n-type
silicon together. Suppose you join a piece of n-type silicon (with
slightly too many electrons) to a piece of p-type silicon (with
slightly too few). What will happen? Some of the extra electrons in
the n-type will nip across the join (which is called a junction) into the holes in the p-type
so, either side of the junction, we'll get normal silicon forming
again with neither too many nor too few electrons in it. Since
ordinary silicon doesn't conduct electricity, nor does this junction.
Effectively it becomes a barrier between the n-type and p-type
silicon and we call it a depletion zone
because it contains no free electrons or holes:
Suppose you connect a battery to this little p-type/n-type junction.
What
will happen? It depends which way the battery is connected. If you
put it so that the battery's negative terminal joins the n-type
silicon, and the battery's positive terminal joins the p-type
silicon, the depletion zone shrinks drastically.
Electrons and holes move across the junction in opposite
directions and a current flows. This is called forward-bias:
However, if you reverse the current, all that happens is that the
depletion zone gets wider. All the holes push up toward one end, all
the electrons push up to the other end, and no current flows at all.
This is called
reverse-bias:
That's how an ordinary diode works and why it allows an electric
current will flow through it only one way. Think of a diode as an
electrical
one-way street. (Transistors,
incidentally, take the junction idea a step further by
putting three different pieces of semiconducting material side by side
instead of two.)
How LEDs work
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.
LEDs are specifically designed so they make light of a certain
wavelength and they're built into rounded plastic bulbs to make this
light more concentrated and brighter. Red LEDs produce light
with a wavelength of about 630-660 nanometres—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. (You can
find out more about the wavelengths of light produced by
different-colored
LEDs on this handy page by OkSolar).
You can also get LEDs that make invisible infrared light, which is
useful in things like "magic eye" beams for optical smoke detectors and
intruder alarms.
Semiconductor lasers work in a similar
way to LEDs but make purer and more precise beams of light.
Photo: LEDs are transparent so light will pass
through them. You can see the two electrical contacts at one end (on the
right) and the rounded lens at the other end. The lens helps the LED to produce a bright, focused beam of light—just like a miniature light
bulb.
These images are available for commercial use from our photo library.
What's so good about LEDs?
In a nutshell:
- 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.
Photo: The red LEDs shining down from the top of this container are
being used to test a way of growing potatoes in space. LEDs are more suitable than ordinary light because
they don't produce heat (which would make the plants dry out).
The red light these LEDs produce makes the plants photosynthesize (produce growth from light and water) more efficiently.
Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
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