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Contributor Notes
  1. What is light-emitting diode (LED)? - Definition from
  2. What is a Light Emitting Diode?
  3. How to Use Them
  4. Light-emitting Diodes Can Replace High-pressure Sodium Lighting for Cut Gerbera Production

This is the tiny LED indicator lamp from a computer printer's control panel. Photo: LEDs are much smaller than lamp bulbs 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. Put many diodes together and you can make as much light as a conventional bulb but use only a fraction as much energy.

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?

What is light-emitting diode (LED)? - Definition from

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 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. Artwork: N-type silicon has extra electrons black blobs , while p-type silicon has a lack of electrons that we can think of as "extra holes" white blobs.

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.

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  • Light-Emitting Diodes (LEDs) -!

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:. 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 :.

What is a Light Emitting Diode?

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. 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. Photo: LEDs are transparent so light will pass through them. You can see the two electrical contacts at one end 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. 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 — nanometers—which happens to look red when we see it, while blue LEDs produce light with shorter wavelengths of about — 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.

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  4. You can also get LEDs that make invisible infrared light, which is useful in things like "magic eye" beams that trigger photoelectric cells in things like optical smoke detectors and intruder alarms. Semiconductor lasers work in a similar way to LEDs but make purer and more precise beams of light. Whom should we thank for this fantastic little invention?

    Nick Holonyak : he came up with the idea of the light-emitting diode in while he was working for the General Electric Company. You might like to watch a short 4-minute video about Nick Holonyak's life and work and his thoughts about the future of LEDs courtesy of the Lemelson Foundation ; if you're feeling more technically minded, you can read all about the solid-state physics behind LEDs in the patents listed in the references below.

    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. Artwork: The structure of an early design for a blue-light LED built from multiple layers on a sapphire Al 2 O 3 substrate. White LEDs, which have achieved practical realization through the use of high-brightness blue LEDs developed in based on Gallium Nitride, are attracting increased attention as a 4th type of light source.

    Applying a sufficient forward voltage will cause the electrons and holes to recombine at the P-N junction, releasing energy in the form of light. Two types of LEDs are available, a lamp type leaded and a chip type surface mount. Users can select the ideal type based on set requirements.

    How to Use Them

    The LED color emission wavelength will change depending on materials used. This makes it possible to customize the color to meet certain wavelength specifications required for applications that use traditional bulbs as light sources for which standards exist , such as traffic lights and automotive lamps. There are several methods of generating white light using LEDs.

    Below is 2 typical emission methods. Combining a blue LED with yellow phosphor, which is a complementary color, will yield white light. This method is easier than other solutions and provides high efficiency, making it the most popular choice on the market. Combining the 3 primary colors will result in white light. Generally this method isn't used for lighting, but for full-color LED devices.

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