Ete411 Lec13

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Lecture on Introduction of Semiconductor at North South University as the undergraduate course (ETE411)
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Dr. Mashiur Rahman
Assistant Professor
Dept. of Electrical Engineering and Computer Science
North South University, Dhaka, Bangladesh
http://mashiur.biggani.org

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Ete411 Lec13

  1. 1. ETE411 :: Lec13<br />Dr. MashiurRahman<br />
  2. 2. Chapter 9 :: Metal-semiconductor junction<br />Contact<br />Semiconductor devices must make contact with the outside world.<br />P-n junction<br />?<br />
  3. 3. History<br />1874- Braun : dependence of the total resistance on the polarity of the applied voltage and on the detailed surface conditions. <br />1904 : Point contact rectifier found in practical applications.<br />1938 – Schottky: potential barrier could arise from stable space charges in the semiconductor alone without the presence of a chemical layer. <br />
  4. 4. Contact<br />Rectifying contacts (Schottky Barrier Diode)<br />Nonrectifying contacts (Ohmic contact)<br />
  5. 5. Energy Band diagram of metal and semiconductor<br />Before contact<br />After contact<br />VB0<br />Фm = metal work function<br />Фs = Semiconductor work function<br />ФB0= ideal barrier height of the semiconductor contact<br />χ= electron affinity<br />Ideal energy-band diagram of a metal-n-semiconductor junction for Фm &gt; Фs<br />
  6. 6. Schottky barrier<br />ФB0= (Фm- χ)<br />Vbi= ФB0-Фn<br />Mechanism : due to the flow of majority carrier electron<br />(Thermionic emission of majority currier)<br />S. M. Sze : Physics of semiconductor Devices (page 255)<br />
  7. 7. Space charge region width :: W<br />Vbi - Va<br />Forward bias<br />The space charge width in a rectifying metal semiconductor contact is universally proportional to the square root of the semiconductor doping. <br />
  8. 8. Measured Schottky-barrier heights (at 300 K)<br />
  9. 9. S. M. Sze : Physics of semiconductor Devices<br />
  10. 10. P-type semiconductor<br />Not included in the course<br />
  11. 11.
  12. 12. χ<br />
  13. 13. Reverse bias and forward bias<br />Reverse bias<br />forward bias<br />
  14. 14. Comparison Schottky Barrier Diode & pn junction Diode<br />Magnitudes of the reverse-saturation current density.<br />Switching characteristics<br />
  15. 15. Magnitudes of the reverse-saturation current density<br />Pnjuction: determined by the diffusion of minority current.  generation current<br />Schottky barrier diode : determined by thermonic emission of majority carriers over a potential barrier. <br />
  16. 16. Turn on voltage<br />Frequency response. <br />Schottky diodes can be used in fast-switching application. <br />
  17. 17. Contact<br />Rectifying contacts<br />Nonrectifying contacts (Ohmic contact)<br />
  18. 18. Nonrectifying contacts (Ohmic contact)<br />Before contact<br />After contact<br />Metal-n-semiconductor junction for Фm &lt; Фs<br />
  19. 19. Voltage applied<br />Positive voltage applied to the metal<br />Positive voltage applied to the semiconductor<br />
  20. 20. Example 9.7 (page 347)<br />
  21. 21. Transport mechanisms<br />Forward bias<br />S. M. Sze : Physics of semiconductor Devices (page 254)<br />Transport mechanisms at metal–semiconductor junctions. (1) Thermionic emission (‘above’ the barrier) (2) tunneling (‘through’ the barrier), (3) recombination in the depletion layer, (4) hole injection from metal<br />
  22. 22. Transport of electrons from the semiconductor over the potential barrier into the metal.  Dominent process for Schottky diodes with moderately doped semicondutor (Si with ND ≤1017cm-3) operated at moderate temperature (room temp.). <br />Thermionic emission<br />
  23. 23. tunneling (‘through’ the barrier)<br />Quantum-mechanical tunneling of electrons through the barrier (important for heavily doped semiconductors and responsible for most ohmic contacts).<br />
  24. 24. recombination in the depletion layer<br />Recombination in the space-charge region <br /> identical to the recombination process in a p-n junction. <br />
  25. 25. hole injection from metal<br />Not included in the course<br />Hole injection from the metal to the semiconductor <br /> Recombination in the neutral region. <br />

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