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Class -12, SEMICONDUCTOR ELECTRONICS, , 1. Metals They possess very low resistivity or high conductivity., ρ ~ 10-2.10-8 Ωm,, σ ~102. 108 Sm-1, 2. Semiconductors They have resistivity or conductivity intermediate to metals, and insulators., ρ ~ 10-5. 106 Ωm,, σ ~ 10+5 .10-6 Sm-1, 3. Insulators They have high resistivity or low conductivity., ρ ~ 1011 . 1019 Ωm,, σ ~ 10-11. 10-19 Sm-1, 4. Energy Band In a crystal due to interatomic interaction, valence electrons of, one atom are shared by more than one atom in the crystal. Now, splitting of, energy level takes place. The collection of these closely spaced energy levels are, called an energy band., 5. Valence Band Valence band are the energy band which includes the energy, levels of the valence electrons., 6.Conduction band is the energy band above the valence band., 7. Energy Band Gap The minimum energy required for shifting electrons from, valence band to conduction band is called energy band gap

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(Eg )., is called energy band gap (Eg )., , onduction band, , 8.Fermi Energy It is the maximum possible energy possessed by free electrons of, a material at absolute zero temperature., 9. On the basis of purity , semiconductors are of two types:, (i) Intrinsic Semiconductors It is a pure semiconductor without any significant, dopant species present, ne = nh =ni, where , ne and nh are number densities of electrons and holes respectively and, ni is called intrinsic carrier concentration., An intrinsic semiconductor is also called an undoped semiconductor or i-type, semiconductor, (ii) Extrinsic Semiconductors Pure semiconductor when doped with the impurity,, it is known as extrinsic semiconductor., Extrinsic semiconductors are basically of two types:, (a) n-type semiconductors, (b) p-type semiconductors

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NOTE: Both the type of semiconductors are electrically neutral., In n-type semiconductor, ✓ majority charge carriers are electrons, ✓ minority charge carriers are holes,, i..e . ne > nh ., (Si or Ge with a pentavalent element, then four of its electrons bond with the, four silicon neighbours, while fifth remains very weakly bound to its parent, atom.), Formation of n-type semiconductor is shown below, , In p-type semiconductor,

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√majority charge carriers are holes, √ minority charge carriers are eletron, , i.e., , nh > ne ., (In a p-type semiconductor, doping is done with trivalent impurity atoms, i.e., those atoms which have three valence electrons in their valence shell.), Formation of p-type semiconductor is shown below: