- A semiconductor is a substance which has resistivity in between conductors and insulators.
- 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.
- The two most frequently used materials are germanium (Ge) and silicon (Si).
- Germanium (Ge) and Silicon (Si) are tetravalent.
- Germanium (Ge) and Silicon (Si) have forbidden energy gap of 0.72 eV and 1.1 eV.
- A vacancy left in the valance band because of lifting of an electron from valance band to conduction band is known as hole.
- An extremely pure semiconductor is known as intrinsic semiconductor.
- A semiconductor to which an impurity at controlled rate is added to make it conductive is known as an extrinsic semiconductor.
- The process by which impurity is added to a semiconductor is known as doping.
- Extrinsic semiconductor may be classified as: (i) n-type semiconductor and (ii) p-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.
- 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.
- When a pn junction conducted across an electric supple, the junction is said to be under biasing.
- 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.
- 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.