The underlying working principle of LED is the energy level transition.
 

There could be several energy levels in the electron of an atom. When the electron bounces from high level to lower level, 

there would be less energy of electron accordingly, and the part that has been gone from electron is transformed to be sent out as photons which a mass of them are so called laser. The LED working theory is similar with it.

But LED is different.
LED does not glow by this electronic leap inside of atom, it is by adding voltage onto the both ends of LED and PN, then the PN junction itself would be an energy level (actually it is a series of energy level), finally, the electron produces photons with leaping in this energy level to glow.

LED refers to Light-Emitting-Diode. It consists of these chemical elements: Ga, As, P, the electron combines with hole to radiate a visible light which could be made as LED light, LED light could be the pilot lamp in circuits and instrument, maybe displayed in the form of character or digit. Gallium Arsenide light glows red light, Gap (gallium phoshpide) light glows green, silicon carbide light glows yellow light.

LED is a sort of semiconductor diode, it could turn the electric energy into luminous energy. LED is as same as general-purpose diode, consisting of PN junction, possessing of unilateral conductivity. Adding direct voltage on LED light, after this, the hole comes from P to N area and the electron coming from N to P would combine with the other hole coming in N 

and P area next to the PN junction within little microns to create fluorescence of spontaneous radiation. The energy state 

of electron and hole is differ from different semiconductor material. The energy released from the combination of electron 

and hole is always different, more releasing energy, the light wavelength would be longer. The red light, green light and 

yellow light is commonly used in LED.

With the constantly development of LED industry, the breakthrough of technology, the strong promotion of application, the 

lighting effect of LED is improved continually, the price has become lower. New combined type of tube core appears which 

would enhance the power of LED constantly. By means of continuously research and development, the break through of new optical design and exploitation of new light kinds, it becomes more hopeful to turn the scale of product single type.

Be aware of the improvement for software, the use of LED would be better  convenient. All these gradual change indicates that LED has its promising prospect in the field of illumination applying.

LED is the fourth generation of light source, it possesses with several characteristics of energy saving, environmental protection, safe, long service life, low power consumption, low heating, high light, waterproof, miniature, shockproof, easy-dimming, concentrated light beam, easy maintaining, and so on, it can be widely used in the field of direction, display, decoration, back light or common illumination.

Advantages: high conversion efficiency of LED lighting ( approaching to 60%, environment protecting, long service time for 

about a hundred thousand hours), low operation voltage (about 3V), less damage for constantly switch on and off, small 

volume, low heat, high light, sturdy and durable, easy dimming, colorful, concentrated light beam and quick started;

Disadvantages: high cost, poor color rendering, inefficiency, constant current drive (needs special drive circuit). By contract, many kinds of traditional illumination have particular defects.