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Question

Addition of pentavalent impurity to a semiconductor creates many


A

Free electrons

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B

Holes

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C

Valence electrons

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D

Bound electrons

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Solution

The correct option is A

Free electrons


  1. The basic prerequisite for the flow of electric current through a body is the presence of free electrons in the body.
  2. A Pure semiconductor (germanium) has four valence electrons.
  3. In the case of a pure semiconductor (germanium), each atom is surrounded by four atoms and there exists a chemical bond between them.
  4. Since there are no free electrons in a pure semiconductor, it cannot conduct electric current.
  5. A pentavalent impurity (arsenic) has five free electrons.
  6. Adding impurities to a pure semiconductor to modify its properties is called “doping”.
  7. When a pentavalent impurity (arsenic) is added to a pure semiconductor, one atom of it forms bonds with four atoms of the pure semiconductor.
  8. Out of the five free electrons of the pentavalent impurity, four are engaged in bond formation and the remaining electron is free and is easily available for conduction.
  9. The pentavalent impurity adds a negative charge carrier (electron) to the pure semiconductor.
  10. The doped semiconductor now possesses a negative charge and is called an “n-type semiconductor”.
  11. Hence, the addition of pentavalent impurities in a semiconductor creates “free electrons” in a pure semiconductor as depicted by the diagram below.
CMS-A-CC-2-4-TH: Basic Electronics Devices and Circuits

Hence, option (A) is correct.


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