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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Semi Conductors Page 1

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1. Semiconductors


Semiconductors are materials whose electrical conductivity is intermediate between that of the good conductors and insulators. Germanium (Ge) and Silicon (Si) are semiconductors which are very useful in electronics.

Structure and Characteristics of Ge and Si:

  • Tetravalent atoms - Both Ge and Si have 4 electrons in the outermost orbit.
  • Each atom forms covalent bonds with FOUR neighbouring atoms as shown below.


Electrical conductivity of a semiconductor can be drastically changed by adding a small quantity of a suitable impurity to molten semiconductor and allowing it to recrystallize. This process of adding impurity to pure semiconductor is known as doping.
Doped semiconductors are of two types:

  1. n-type semiconductor
  2. p-type semiconductor


Mechanism of Hole Movement: In a p-type semiconductor, one of the 4 bonds between an impurity and a germanium/silicon atom, has a vacancy for an electron. This empty space or vacancy is known as a hole. When an electric field is applied to the crystal, an electron from the neighbouring atom jumps into the hole creating a fresh hole in its previous place. This process repeats itself many times and the hole effectively moves towards the negative potential.




Construction and Working of p - n Junction Diode


When a p-type semiconductor is joined together with n-type semiconductor, the boundary between them is called p-n junction.

If a germanium or silicon crystal is doped in such a way that half of it is
p-type and the other half is n-type then we get a p-n junction diode. The p side forms the anode and the n side forms the cathode of the diode.


Because of diffusion at the junction, few electrons go from n region to p region. As a result, n-region is at a small positive potential with respect to p region. Similarly few holes go from p region to n region. As a result, p region is at small negative potential with respect to n region.

The region near the junction (thickness about 10
-6m) is depleted of holes and electrons and it is called depletion region. The result of build up of positive and negative charges near the junction is to set up a potential difference. It is called a potential barrier.

p-n Junction

  • When a battery is connected to the junction diode with the positive terminal to the p-side and negative terminal to the n side, the p-n junction is said to be forward-biased.

  • In the forward-biased p-n junction, the applied potential difference helps the electrons and holes to cross the junction. It reduces the width of the depletion region.
  • The diode conducts when it is forward-biased.

Reverse-biased p-n Junction

  • When a battery is connected to the junction diode with its positive terminal to the n-side and negative terminal to the p side, the p-n junction is said to be reverse-biased.

  • In a reverse-biased p-n junction, the applied potential opposes the movement of electrons and holes across the junction. The width of the depletion region increases.
  • The diode does not conduct when it is reverse-biased.




Advantages of a Semiconductor Diode (or Semiconductor Devices in General) Over Vacuum Diode


  1. Semiconductor diode is an open structure and it requires no vacuum. The elements of a diode valve must be protected from oxidation by enclosing them in a highly evacuated bulb.
  2. The life of semiconductor diode is long because there is no filament to burn out as in vacuum diodes. The life of filament in vacuum diodes decreases by vibration.
  3. Semiconductor diodes operate on very low voltage while the operating voltage for the vacuum diode is high.
  4. Semiconductor diodes are small in size and light in weight. Vacuum diodes are bulky and heavier.
  5. The action of a semiconductor diode is instantaneous when current is switched on. It is not so in vacuum diode where the filament takes some time to get heated and to emit electrons.
  6. Semiconductor diodes are less expensive compared to valve diodes.




Comparison of Semiconductor Diode and Vacuum Diode



Semiconductor Diode

Vacuum Diode

  1. It operates at very low voltage.
  2. Power consumption is very low.
  3. It starts instantaneously.
  4. Conduction charges are present in the solid hence no heat energy is used for the emission of electrons.

  5. Because of electric conduction through solid, there are no problems of leakage.
  6. It occupies less space and light in weight.
  7. Semiconductor devices are more economical.
  1. It needs very high operating voltage.
  2. Power consumption is very high.
  3. It does not start instantaneously.
  4. For the conduction through vacuum, electrons must be emitted from the cathode and hence heat energy is used.
  5. It needs an evacuated tube hence there are problems of leakage.

  6. It is bulky and heavy.

  7. The vacuum tube devices are costlier.



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