E-StoreSemi conductors
6 Jan 2008 20:00:29 IST
Semi conductors
THey are solids where there is only a small difference in energy called band gap,between the filled valency band of electrons and a conduction band.The conductivity is in between that of a metal and an insulator and depends on the no. of electrons in conduction band .
MOST important semi conductors widely used is silicon and germanium.Atoms of Si and Ge have 4 elns in their outer shell which can form 4 covalent bonds to other atoms.
Pure Si and Ge can be made semiconducting in controlled way by adding impurities which acts as charge carriers..Some atoms which have 5 electrons like Arsenic are deliberately added to silicon .This is called DOPPING .A minute portion of Si is replaced by As atom with 5 electns.only 4 of 5 is required to form bonds in lattice. THis extra electron can carry current.THis type of semiconductors is called n-type......NEGETIVE type
the other type is p-type or POsitive type.where the Si is dopped with atoms of 3 outer electrons like Indium.It uses its 3 outer electrons to form 3 bonds in the lattice, but one bond is incomplete .This site is called positive hole.The positive hole can migrate across the crystal conducting electricity
Generally Arsenic and Indium are used as dopping materials becoz of thier low melting points
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