Ete411 Lec13


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

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Ete411 Lec13

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