LED DIODE

            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 diagram summarizes what happens:

1.   N-type silicon (red) has extra electrons (black).

2.   P-type silicon (blue) has extra holes (white).

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.

                     
                                                Types of LEDs

       
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. (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 invisibleinfrared light, which is useful in things like "magic eye" beams that trigger photoelectric in things like optical smokedetectors 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.

                    
                                                                       invented LEDs

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 about nick holonyaks 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.

                     LED COLOR

Typical LED Characteristics
Semiconductor Material	Wavelength	Colour	VF @ 20mA
GaAs	850-940nm	Infra-Red	1.2v
GaAsP	630-660nm	Red	1.8v
GaAsP	605-620nm	Amber	2.0v
GaAsP:N	585-595nm	Yellow	2.2v
AlGaP	550-570nm	Green	3.5v
SiC	430-505nm	Blue	3.6v
GaInN	450nm	White	4.0v

Thus, the actual colour of a light emitting diode is determined by the wavelength of the light emitted, which in turn is determined by the actual semiconductor compound used in forming the PN junction during manufacture.

Therefore the colour of the light emitted by an LED is NOT determined by the colouring of the LED’s plastic body although these are slightly coloured to both enhance the light output and to indicate its colour when its not being illuminated by an electrical supply.

Light emitting diodes are available in a wide range of colours with the most common being red , amber,yellow  and green and are thus widely used as visual indicators and as moving light displays.

Recently developed blue and white coloured LEDs are also available but these tend to be much more expensive than the normal standard colours due to the production costs of mixing together two or more complementary colours at an exact ratio within the semiconductor compound and also by injecting nitrogen atoms into the crystal structure during the doping process.

From the table above we can see that the main P-type dopant used in the manufacture of Light Emitting Diodes is Gallium (Ga, atomic number 31) and that the main N-type dopant used is Arsenic (As, atomic number 33) giving the resulting compound of Gallium Arsenide (GaAs) crystalline structure.

The problem with using Gallium Arsenide on its own as the semiconductor compound is that it radiates large amounts of low brightness infra-red radiation (850nm-940nm approx.) from its junction when a forward current is flowing through it.

The amount of infra-red light it produces is okay for television remote controls but not very useful if we want to use the LED as an indicating light. But by adding Phosphorus (P, atomic number 15), as a third dopant the overall wavelength of the emitted radiation is reduced to below 680nm giving visible red light to the human eye. Further refinements in the doping process of the PN junction have resulted in a range of colours spanning the spectrum of visible light as we have seen above as well as infra-red and ultra-violet wavelengths.

By mixing together a variety of semiconductor, metal and gas compounds the following list of LEDs can be produced.