Chemistry: Solid State: Doping of Semiconductors: Electron-Rich and Deficit Impurities (For CBSE, ICSE, IAS, NET, NRA 2022)

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Doping of Semiconductors

  • The conductivities of silicon and germanium are too low for practical uses.
  • Their conductivities can be improved by introduction of controlled quantities of impurities which are either electron-rich or electron-deficit with respect to these elements. This process is known as doping

Doping with Electron – Rich Impurities

  • Silicon and germanium both belong to group 14 of the periodic table and have 4 valence electrons each.
  • In their crystal lattice each silicon atom forms four covalent bonds with its neighbors as shown in picture below.
Electron Rich Impurities
  • When it is doped with a group 15 element like As or P, where each As atom carries 5 valence electrons which is one extra than Si.
  • After sharing its four electrons with four neighboring Si atoms, it has 9 electrons in its valence shell which is highly unstable electronic configuration.
Electron Rich Impurities
  • The ninth electron being highly unstable moves freely in the whole of the crystal lattice.
  • In the presence of an electric field this mobile electron moves from negative to positive terminal and increases the conductivity of Si or Ge.
  • Since the increased conductivity of Si or Ge is due to negatively charged mobile electrons, the Si doped with electron rich impurity becomes a n-type semiconductor.

Doping with Electron-Deficit Impurities

  • When Si or Ge is doped with a group 13 element like B or Al containing 3 valence electrons results in the formation of one-electron vacancy in the structure which is called an electron hole.
  • An electron from its neighboring atom can occupy it which leaves a hole at its original position.
  • The electrons can randomly occupy holes and the hole would appear to move randomly.
  • On applying an electric field, the electrons move from negative to positive terminal and the hole would move in the opposite direction, i.e.. , from positive to negative terminal and would behave as positively charged. Such semiconductors are called p-type semiconductors.
Electron Deficit Impurities
  • The movement of electrons and the hole increases the conductivity of Si.
  • Semiconductors like Si doped with electron-deficit or electron-rich impurities are called extrinsic semiconductors.

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