The LED is an acronym of Light Emitting Diode; it generates light when electrons in conduction band recombine with holes in the valence band. The LEDs are made up of semiconductors such as GaAsP and GaP, the semiconductor material such as silicon and germanium are not used in the construction of LED.
The semiconductor such as silicon and germanium release energy in the form of heat when holes and electron recombine but we want energy in the form of light to operate LED. The semiconductor such as Gallium Arsenide, Gallium Phosphide release energy in the form of light during recombination of electrons and holes. This process is called electroluminescence.
Thus, LED converts the electrical energy into light energy. The conversion efficiency of LEDs is much higher than that of the tungsten lamp. The LEDs are used for display devices but the power requirement is quite large. For displaying a single digit the LED require 10-250 mW of power. The brightness level of LED display is very high thus, it provides a good quality display.
LCD is an acronym for Liquid crystal display; the nematic crystal is used for the construction of LCD. The nematic crystal is the liquid crystal, the liquid crystal can be considered as the fourth state of the matter. This is because the liquid crystal has the property of solid as well as liquid. The crystal structure is responsible for the solid characteristics of liquid while the ability of the liquid crystal to flow imparts it the property of the liquid.
The LCD consists of a light source which supplies light from external source to the liquid crystal. The liquid crystals are filled between the polarizing films. These polarizers are applied voltage from the external electric field. The voltage applied through external electric field modifies the liquid crystal in such a way that the light is absorbed by the rear polarizer and this light creates dark and bright spots on the screen.
The liquid crystal is fed between the front polarizer and rear polarizer, when the voltage is not applied from the external electric field then the light ray rotate at the right angle and is reflected back to the external source. To prevent this polarizer are coated with conducting material and the electric field is applied to it.