A LED (Light Emitting Diode) is an optoelectronic device manufactured with semiconductor material forming pn junction, emitting light when is directly polarized, electric current flowing through it. The electrons are able to recombine with holes within the device, thus moving to a lower energy level and releasing energy as photons. This effect is electroluminescence. See Figure 2.
The wavelength of light emitted, and therefore the color, is determined by the energy of the bandgap of the materials forming the pn junction. Therefore, the color of the light emitted from an LED depends of the semiconductor material that is manufactured.
The LED is a diode, it consists of two terminals: anode (P-type material) and cathode (N-type material). Current can flow in the forward direction (from anode to cathode) but not reverse. When current is flowingt in forward direction by the LED, electrons cross the potential barrier, and recombination occurs with holes, this phenomenon emitting photons, then it is reasonable to think that if we increase the current flowing through an LED, this issue more photons, but also increase the temperature, hence lower efficiency and lower life.
The voltage (Vd) of a LED varies according to the semiconductor material that is made, then the potential difference anode - cathode of a red LED is about 1.7 V and a UV LED is about 4 V.
Usually encapsulated in an epoxy plastic cover greater strength than glass. The plastic can be colored but does not affect the color of light emitted, but is used to identify the LED. The cover of a LED has a lens that determines the angle of emission of the LED.