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Semiconductor Electronic: Material, Devices And Simple Circuits — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsSemiconductor Electronic: Material, Devices And Simple Circuits4 Marks
Question

Light emitting diode is a photoelectric device which converts electrical energy into light energy. It is a heavily doped p-n junction diode which under forward biased emits spontaneous radiation. The general shape of the I-V characteristics of an LED is similar to that of a normal p-n junction diode, as shown. The barrier potentials are much higher and slightly different for each colour.

  1. The I-V characteristic of an LED is:

  1. The schematic symbol of light emitting diode is (LED).

  1. An LED is constructed from a p-n junction based on a certain Ga-As-P semiconducting material whose energy gap is 1.9eV. Identify the colour of the emitted light.
  1. Blue.
  2. Red.
  3. Violet.
  4. Green.
  1. Which one of the following statement is not correct in the case of light emitting diodes?
  1. It is a heavily doped p-n junction.
  2. It emits light only when it is forward biased.
  3. It emits light only when it is reverse biased.
  4. The energy of the tight emitted is less than the energy gap of the semiconductor used.
  1. The energy of radiation emitted by LED is:
  1. Greater than the band gap of the semiconductor used.
  2. Always less than the band gap of the semiconductor used.
  3. Always equal to the band gap of the semiconductor used.
  4. Equal to or less than the band gap of the semiconductor used.
✓

Answer

  1. (b)

Explanation:

The I-V characteristics of an LED is similar to that of a Si junction diode. But the threshold voltages are much higher and slightly different for each colour.

  1. (b)

  1. (b) Red.

Explanation:

As $\text{E}_\text{g}=\frac{\text{hc}}{\lambda}$

$\therefore\lambda=\frac{\text{hc}}{\text{E}_\text{g}}$

Here, Eg = 1.9ev, hc = 1240eVnm

$\therefore\lambda=\frac{1240\text{eVnm}}{1.9\text{eV}}=652.6\text{nm}$

Hence, the emitted light is of red colour.

  1. (c) It emits light only when it is reverse biased.

​​​​​​​Explanation:

A light emitting diode is a heavily doped p-n junction diode which emits light only when it is forward biased.

  1. (d) Equal to or less than the band gap of the semiconductor used.

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