LEDs are solid-state semiconductor devices that convert electrical energy directly into light. Thermal sources of light such as flames and incandescent filaments emit light when heated, either by chemical reaction (flames) or electrical heating (filament lamps). LED "cold" generation of light leads to high efficacy because most of the energy radiates within the visible spectrum.

Other common high efficiency light sources, such as fluorescent lamps and electro luminescent devices, also produce light without much thermal radiation outside the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable; they also provide longer lamp life than other sources.

Light is generated inside the chip, a solid crystal material, when current flows across the junctions of different material compositions.

An LED consists of two elements of processed material called P-type semiconductors and N-type semiconductors. These two elements are placed in direct contact, forming a region called the P-N junction. The composition of the different materials determines the wavelength and therefore the colour of light generated LED resembles most other diode types, but there are important differences. The LED has a transparent package, allowing visible or IR energy to pass through. Also, the LED has a large PN-junction area whose shape is tailored to the application. The device shown in Figure 1 is an AlGaInP LED because the semiconductor layers are aluminum (Al), gallium (Ga), indium (In), and phosphate (P).

Main LED materials
The main semiconductor materials used to manufacture LEDs are:
*Indium gallium nitride (InGaN): blue, green and ultraviolet high-brightness LEDs
*Aluminum gallium indium phosphide (AlGaInP): yellow, orange and red high-brightness LEDs
*Aluminum gallium arsenide (AlGaAs): red and infrared LEDs
*Gallium phosphide (GaP): yellow and green LEDs