12th physics. NCERT notes

Light Emitting Diode (LED)

A light emitting diode (LED) is a forward biased PN junction diode, which emits visible light when energized.

When a junction diode is forward biased, electrons from N-side and holes from P-side move towards the depletion region and recombination takes place. When an electron in the conduction band recombines with a hole in the valence band, energy is released. The LED is turned ON, when it is forward biased and it is turned OFF, when it is reverse biased. The color of the emitted light will depend upon the type of the material used. Fig shows the symbol of LED. LEDs are used for instrument displays, calculators and digital watches.

Light Emitting Diode (LED)

LEDs have the following advantages over conventional incandescent low power lamps:

  • Low operational voltage and less power.
  • Fast action and no warm-up time required.
  • The bandwidth of emitted light is 100 Å to 500 Å or in other words, it is nearly (but not exactly) monochromatic.
  • Long life and ruggedness.
  • Fast on-off switching capability


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