 | The short name for Light Emitting Diode is LED. One single led has an anode and a cathode trough which a current flows and a led chip which is located in a reflector. These parts together are wrapped with a transparent or colored synthetic cover and form a small lens that bundles the emitting light (see picture below) |
The color of the light dependents on the nature of the material used inside the led chip. A led consists of two semi-conductors that have both different electron paths that correspond to their own energy level. When the led is energized, a current starts to flow. Now, electrons are transported via the cathode and move at a certain moment to a lower electron path of the other material. This process deserves an additional explanation.
The fact is that several energy levels exist inside the different materials. Billions of atoms are located very closely together and start to form energy paths. Between these paths a bandgap is situated. The highest energy level by which electrons are still present at the absolute zero point (0 kelvin) is called the Fermi-level. At this temperature all levels below the Fermi-level are taken and all levels above the Fermi-level are free. In order to move, the electrons need extra kinetic energy. In this situation it is not possible to get this energy and therefore, they are not able to move and a current flows hardly. When the Fermi-level is located in the restricted zone, the highest path below the Fermi level is called the valence band and the lowest path above the Fermi-level is called the conduction band. This is also the case for a semi-conductor, like the key material of the led (see picture below).
When the restricted zone is not that big, electrons can easily move from the valence band over the restricted zone into the conduction band, because it absorbs energy from outside. During this transition to a lower electron path the electron releases energy in the form of a photon and emits light.
The nature of the material inside the led causes the difference of energy between the electron paths and therefore the color that the led emits. The pixels of led displays are composed with the colors Red, Green and Blue (RGB) and are obtained with the following materials:
Red: Gallium-aluminiumarsenide (AlGaAs)
Green: Galliumfosfide (GaP)
Blue: Siliciumcarbide (SiC)
Difference between LCD and LED
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