Class 12 Physics Semiconductor Electronics Light emitting diode

Light emitting diode

Application – They convert electrical energy into light

  • It is a heavily doped p-n junction which under forward bias emits spontaneous radiation
  • The diode is encapsulated with a transparent cover so that the emitted light can come out
  • When the diode is forward biased, the electrons are sent from n layer to p layer and the holes are sent from p to n
  • Thus, at the boundary due to forward bias, the concentration of the minority carriers increases
  • The excess minority carriers recombine with the majority carriers, near the junction
  • On recombination, energy is released in the form of photons
  • Photons with energy equal to or slightly less than the band gap is emitted
  • When the forward current of the diode is small, the intensity of the light emitted is small
  • As the forward current increases, the intensity of light increases and reaches maximum.
  • Further increase in forward current, results in the decrease of light intensity
  • LEDs are thus biased in such a way such that the efficiency is maximum
  • The reverse breakdown voltage of LED is very small (say) 5V. Proper precaution should be taken such that high reverse voltage do not appear across them
  • LEDs have the following advantages over the conventional lamps:
    • Low operational voltage and less power
    • Fast action and no warm-up time required
    • Long life
    • Fast on-off switching capacity

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