Covalent Bonds,Electrons and Doping

COVALENT BONDS

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds. In short, attraction-to-repulsion stability that forms between atoms when they share electrons is known as covalent bonding.

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In this figure of Silicon each atom shares an electron with each of surrounding atoms, creating a covalent bond surrounding atoms are bonded to other atoms.

Covalent bonds in silicon materials give them crystal like structure 8 electrons bonded around each atom.

CONDUCTION ELECTRONS IN PURE SILICON

•    Heat is a form of energy.
•    At normal room temperature, electrons in VB get heat energy and can jump to CB.
•    These free electrons are conduction electrons.

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•    Electron hole pairs are generated by breaking of covalent bonds.

RECOMBINATION OF HOLES AND ELECTRONS

Recombination will take place in VB under two conditions.
•    When a free electron in CB looses its energy due to cold it will fall back to VB and re combine with a hole.
•    An electron in VB gets a little energy, then can move to into near by hole but it will leave a new hole
So it is a hole current or hole-movement in VB.

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DOPING

Adding impurities in pure semiconductor is called doping or to convert intrinsic semiconductor to extrinsic semiconductor.

•    Adding a pentavalent atom in pure semiconductor is called doping.
•    Adding trivalent atom in pure semiconductor is called doping
•    Doping convert intrinsic semiconductor into extrinsic semiconductor.

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