BAND THEORY OF SOLIDS

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BAND THEORY OF SOLIDS

  1. 1. 9/2/2012 weyes57 1
  2. 2. 9/2/2012 weyes57 2
  3. 3. Content 1.Introduction 2.Semiconductors 3.Energy band in solids 4. Conduction band, Valance band and Forbidden gap 5.Insulators, Conductors and Semiconductors 6.Electrons and holes in semi conductors 7.Intrinsic semiconductor 8.Doping of a semiconductor 9.Extrinsic semi conductors 10.N-type semi conductor 11.P-Type semiconductor9/2/2012 weyes57 3
  4. 4. In the year 1947, INTRODUCTIONTransistor wasinvented by threescientists.William Shockley, JohnBardeen and WalterBrattain in 1956 ,shared Nobel Prize inphysics for creating thetransistor. 9/2/2012 weyes57 4
  5. 5. SEMICONDUCTORS Semiconductors have resistivity between good conductors and insulators. The resistivity of semiconductor lies approximately in between 10 -2 ohm m and 10 4 ohm m at room temperature. .9/2/2012 weyes57 5
  6. 6. ENERGY BANDS IN SOLIDS There are discrete energy levels in the case of an isolated atom.9/2/2012 weyes57 6
  7. 7. Arrangement of electrons in an isolated Silicon atom9/2/2012 weyes57 7
  8. 8. In solids , the atoms are arranged in a systematic space lattice and each atom is influenced by neighbouring atoms. The closeness of atoms results in the intermixing of electrons of neighbourring atoms.9/2/2012 weyes57 8
  9. 9. Insulators Conduction Band In an insulator, the forbidden gap is very large and in general is more than 3eV. FORBIDDEN GAP No electron is available for conduction.Energy Valence Band Large amount of energy is needed to move electron from valance band to Filled Band conduction band. 9/2/2012 weyes57 9
  10. 10. In the case of semiconductors the Semiconductors forbidden gap is very small. At 0K the conduction band is empty and the valence band is Conduction Band completely filled. FORBIDDEN GAP Around 0.7eV (Ge) and When a small amount of energy is 1.1 eV (Si) supplied, the electrons can easily jump the forbidden gap.Energy Valence Band The conductivity of a semiconductor is of the order of 10 2mho m-1 Filled Band 9/2/2012 weyes57 10
  11. 11. Conductors In conductors there is no forbidden gap. The valence band and the conduction band overlap. Conduction Band The electrons from valence band freely enter into the conduction band due to overlapping of bands.Energy No forbidden gap Therefore very low potential Valence Band difference can cause continuous flow of current. 9/2/2012 weyes57 11
  12. 12. Electrons and holes in semiconductors At absolute 0 temperature, At 0 K in a pure semiconductor the valence band is Conduction Band completely filled and theEnergy conduction band is vacant. Valence Band Electron9/2/2012 weyes57 12
  13. 13. Electrons and holes in semiconductors At Room At room temperature temperature some of the electrons get Conduction Band energy to break the covalent bond and moves Energy in to the conduction band. Valence Band Electron Hole9/2/2012 weyes57 13
  14. 14. FORMATION OF N - TYPE MATERIAL9/2/2012 weyes57 14
  15. 15. Group 3 atoms have one less electron than silicon so when they are introduced into the crystal, there is a ‘hole where one extra electron should be. Electrons from neighbouring atoms can move into this hole, leaving a hole where they used to be which is in turn filled by another neighbouring electron.9/2/2012 weyes57 15
  16. 16. FORMATION OF P - TYPE MATERIAL9/2/2012 weyes57 16
  17. 17. 9/2/2012 weyes57 17
  18. 18. 9/2/2012 weyes57 18

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