In a light-emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "electroluminescence". The wavelength of the light depends on the energy band gap of the semiconductors used. Contact online >>
In a light-emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "electroluminescence". The wavelength of the light depends on the energy band gap of the semiconductors used.
The "Light Emitting Diode" or LED as it is more commonly called, is basically just a specialised type of diode as they have very similar electrical characteristics to a PN junction diode. This means that an LED will pass current in its forward direction but block the flow of current in the reverse direction.
LEDs (that''s "ell-ee-dees") are a particular type of diode that convert electrical energy into light. In fact, LED stands for "Light Emitting Diode." (It does what it says on the tin!) And this is reflected in the similarity between the diode and LED schematic symbols: In short, LEDs are like tiny lightbulbs.
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 or a Light Emitting Diode is semiconductor device that emits light due to Electroluminescence effect. An LED is basically a PN Junction Diode, which emits light when forward biased.
Learn how LEDs work, how they are made, and what colours they emit. See examples of LEDs and their applications in TVs, displays, and indicators.
Photo: Unlike incandescent lamp bulbs (used in things like flashlights), which burn out relatively quickly, LEDs are extremely reliable—so much so, that they''re typically soldered right onto electronic circuit boards. They virtually never wear out! This is the tiny LED indicator lamp from a computer printer''s control panel.
If you know a bit about electricity,you''ll know that materialsfall broadly into two categories. There are some that let electricityflow through them fairly well, known as conductors,and othersthat barely let electricity flow at all, known as insulators.Metalssuch as copper and gold are examples of good conductors, whileplastics and wood are typical insulators.
What''s the difference between a conductor and an insulator?Solids are joined together when their atomslink up. In something like a plastic, the electrons in atoms are fully occupied bindingatoms into molecules and holding the molecules together. They''re notfree to move about and conduct electricity. But in a conductor theatoms are bound together in a different kind of structure. In metals,for example, atoms form a crystalline structure (a bit like equal-sized marblespacked inside a box) and some of their electrons remain free tomove throughout the whole material, carrying electricity as they go.
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 and still save energy.
Not everything falls so neatly into the two categories ofconductor or insulator. Put a big enough voltage across any materialand it will become a conductor, whether it''s normally an insulator ornot. That''s how lightning works. When a cloud moves through the airpicking up electric charge, it creates a massive voltage betweenitself and the ground. Eventually, the voltage is so big that theair between the cloud and the ground (which is normally an insulator)suddenly "breaks down" and becomes a conductor—and you get amassive zap of lightning as electricity flows through it.
Certain elements found in the middle of the periodictable (the orderly grouping of chemical elements) arenormally insulators, but we can turn them into conductors with achemical process called doping.We call these materials semiconductors and silicon and germanium aretwo of the best known examples.Silicon is normally an insulator, but if you add afew atoms of the element antimony, you effectively sprinkle in someextra electrons and give it the power to conduct electricity. Siliconaltered in this way is called n-type (negative-type) becauseextra electrons (shown here as black blobs) can carry negative electriccharge through it.
In the same way, if you addatoms of boron, you effectively take away electrons from the siliconand leave behind "holes" where electronsshould be. This type ofsilicon is called p-type (positive type) because the holes (shown hereas white blobs) can movearound 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-typesilicon together. Suppose you join a piece of n-type silicon (withslightly too many electrons) to a piece of p-type silicon (withslightly too few). What will happen? Some of the extra electrons inthe n-type will nip across the join (which is called a junction) into the holes in the p-typeso, either side of the junction, we''ll get normal silicon formingagain with neither too many nor too few electrons in it. Sinceordinary silicon doesn''t conduct electricity, nor does this junction.Effectively it becomes a barrier between the n-type and p-typesilicon and we call it a depletion zonebecause it contains no free electrons or holes:
Suppose you connect a battery to this little p-type/n-type junction.Whatwill happen? It depends which way the battery is connected. If youput it so that the battery''s negative terminal joins the n-typesilicon, and the battery''s positive terminal joins the p-typesilicon, the depletion zone shrinks drastically.Electrons and holes move across the junction in oppositedirections and a current flows. This is called forward-bias:
However, if you reverse the current, all that happens is that thedepletion zone gets wider. All the holes push up toward one end, allthe 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 electriccurrent will flow through it only one way. Think of a diode as anelectricalone-way street. (Transistors,incidentally, take the junction idea a step further byputting three different pieces of semiconducting material side by sideinstead of two.)
LEDs are simply diodes that are designed to give off light. When adiode is forward-biased so that electrons and holes are zipping backand forth across the junction, they''re constantly combining andwiping one another out. Sooner or later, after an electron moves fromthe n-type into the p-type silicon, it will combine with a hole anddisappear. That makes an atom complete and more stable and it givesoff a little burst of energy (a kind of "sigh of relief") in theform of a tiny "packet" or photon of light.
This diagram summarizes what happens:
Photos (above and below): LEDs are transparent so light will passthrough them. You can see the two electrical contacts at one endand 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 certainwavelength and they''re built into rounded plastic bulbs to make thislight brighter and more concentrated. Red LEDs produce lightwith a wavelength of about 630–660 nanometers—which happens to lookred when we see it, while blue LEDs produce light with shorterwavelengths of about 430–500 nanometers, which we see as blue. You can also get LEDs that make invisible infrared light, which isuseful in things like "magic eye" beams that triggerphotoelectric cellsin things like optical smoke detectors andintruder alarms. Semiconductor lasers work in a similarway to LEDs but make purer and more precise beams of light.
Photo: Colored LEDs produce different wavelengths of light.Red LEDs produce longer wavelengths of about 630–660 nanometers (nm), yellow come in at 555–600nm, and green are shorter again at about 515–555nm. Outside the visible spectrum, infrared LEDs makewavelengths greater than about 1000nm, while ultraviolet LEDs are often shorter than 400nm.
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. You might like to watch a short (4-minute) video aboutNick Holonyak''s life and work and his thoughts aboutthe 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 becausethey 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 Marshall Space Flight Center (NASA-MSFC) and Internet Archive.
Another new development in this area is the microLED display, made from very bright, very efficient LEDs with much higher pixel density than the screens currently used in things like laptops and smartphones. Although microLEDs are still a very new technology, they''re expected to find applications in things like heads-up displays for virtual reality and augmented realityand smartphone screens that use much less battery power.
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