Light energy can excite an electron into the conduction band. On the other band, when recombination takes place energy is released. In a semiconductor with an indirect band gap, e.g, silicon,germanium etc. recombination releases heat to the lattice. However, in some materials (e.g gallium arsenide), direct transitions occur between conduction band and transition band. In these materials, the radiation is in the visible and infrared regions. This effect, knows as injection electroluminescence, finds important application in digital watches, calculators etc. In a light emitting diode, an electric current causes the injection of minority carriers into regions of the crystal where they can recombine with majority carriers resulting in emission of light. The light output varies as (current)n , where n varies from 1.2 to 1.5 A wide range of photon energies extending from ultraviolet to infrared are available. The available energies can be increased still further by mixing the compounds. Commonly used displays are red, yellow,green and orange.
Figure-1 shows symbol for LED
Figure-2 shows a simple circuit to regulate the current through an LED. The forward current If
Figure-1 shows symbol for LED
Figure:01 LED Symbol
Figure:02 LED circuit diagram
Figure-2 shows a simple circuit to regulate the current through an LED. The forward current If
If = (V-Vf / R)
Where Vf is the forward voltages across the LED. The LEDs for different colours have different values of Vf and If. The diode D1 is to protect the LED against reverse breakdown voltage. When reverse voltage exceeds a certain value, D1 starts conducting and protect the LED.
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