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.
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.
• 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.
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.