1. A semiconductor is a substance which has resistivity in between conductors and insulators.
  2. Germanium, silicon, selenium, carbon etc. are the example of semiconductor materials.

Properties of semiconductors

  • The resistivity of a semiconductor is less than insulator but more than a conductor.
  • Semiconductors have negative temperature coefficient of resistance.
  • Semiconductors formed by covalent bonds.
  • When a suitable impurity is added to a semiconductor its current conducting properties change appreciably.
  1. The two most frequently used materials are germanium (Ge) and silicon (Si).
  2. Germanium (Ge) and Silicon (Si) are tetravalent.
  3. Germanium (Ge) and Silicon (Si) have forbidden energy gap of 0.72 eV and 1.1 eV.
  4. A vacancy left in the valance band because of lifting of an electron from valance band to conduction band is known as hole.
  5. An extremely pure semiconductor is known as intrinsic semiconductor.
  6. A semiconductor to which an impurity at controlled rate is added to make it conductive is known as an extrinsic semiconductor.
  7. The process by which impurity is added to a semiconductor is known as doping.
  8. Extrinsic semiconductor may be classified as: (i) n-type semiconductor and (ii) p-type semiconductor.

N-type semiconductor

  • When a small amount of pentavalent impurity is added to a pure semiconductor providing a large number of free electrons in it, the extrinsic semiconductor thus formed is known as n-type semiconductor.
  • Arsenic (atomic number 33) and antimony (atomic number 51) are the example of pentavalent impurity.
  • Pentavalent impurities are also known as donor impurities.
  • In n-type semiconductors electrons are majority carriers and holes are minority carriers.

P-type semiconductor

  • When a small amount of trivalent impurity is added to a pure semiconductor providing a large number of holes in it, the extrinsic semiconductor thus formed is known as p-type semiconductor.
  • Gallium (atomic number 31) and indium (atomic number 49) are the example of trivalent impurity.
  • Trivalent impurities are also known as acceptor impurities.
  • In n-type semiconductors holes are majority carriers and electrons are minority carriers.
  1. When a pn junction conducted across an electric supple, the junction is said to be under biasing.
  2. When the positive terminal of a dc source or battery is connected to p-type and negative terminal is connected to n-type of a pn junction the junction is said to be in forward biasing.
  3. When the negative terminal of a dc source or battery is connected to p-type and positive terminal is connected to n-type of a pn junction the junction is said to be in reverse biasing.