Question 17: What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.
Answer:
Semi- conductors are the solids that have properties intermediate between metals and insulators.They have only small difference in energy between the filled valence band and empty conduction band.The conductivity is also intermediate between that of a metal and an insulator and depends upon the number of electrons in the conduction band.
The semiconductors behaves in two types as
a) Intrinsic conduction
b) Extrinsic conduction
(a) Intrinsic conduction = pure silicon and germanium are poor conductors because they have network of four strong covalent bonds. But when electricity is provided to the crystals, the electrons moves leaving behind a positive charge or hole at the site of missing electron, as a result of which the crystal will now conduct electricity.
(b) Extrinsic conduction = here the conductivity of intrinsic conductors is increased by adding an appropriate amount of suitable impurity. This process is called doping. The impurities are of 2 types:
(1) Electron rich
(2) Electron deficient
(1) Electron rich impurity – here atoms with five valence electrons are added to the semiconductor, as a result of which four out of five electrons are used in formation of four covalent bonds with four neighbouring silicon atoms. The fifth electron is extra and becomes delocalised. These delocalised electrons increase the conductivity of doped silicon, hence increases the conductivity. This type of conductor is called n-type semiconductor.
(2) Electron deficient impurity – here an atom with three valence electrons are added to the semiconductor, as a result of which silicon or germanium atom are replaced by the impurity atom. The valence electrons of impurity will make bond with three atom but fourth atom remains free in the crystal of silicon or germanium, which is now available for conducting electricity. This type of conductor is called p-type semi- conductor.
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