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ECE685 Nanoelectronics – Semiconductor Devices

ECE685 Nanoelectronics – Semiconductor Devices. Lecture given by Qiliang Li. Silicon Structure. Unit cell of silicon crystal is cubic. Each Si atom has 4 nearest neighbors . Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Dopants, Electrons and holes. As. B.

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ECE685 Nanoelectronics – Semiconductor Devices

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  1. ECE685 Nanoelectronics – Semiconductor Devices Lecture given by Qiliang Li

  2. Silicon Structure • Unit cell of silicon crystal is cubic. • Each Si atom has 4 nearest neighbors.

  3. Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Dopants, Electrons and holes As B

  4. Relationship between Resistivity and Dopant Density DOPANT DENSITY cm-3 P-type N-type RESISTIVITY (cm) = 1/

  5. GaAs, III-V Compound Semiconductors, and Their Dopants Ga As Ga As Ga As Ga As Ga · GaAs has the same crystal structure as Si. · GaAs, GaP, GaN are III-V compound semiconductors, important for optoelectronics. · Wich group of elements are candidates for donors? acceptors?

  6. Energy Band Model · Energy states of Si atom (a) expand into energy bands of Si crystal (b). · The lower bands are filled and higher bands are empty in a semiconductor. · The highest filled band is the valence band. · The lowest empty band is the conduction band .

  7. Energy Band Diagram Conduction band E c Band gap E g E v Valence band · Energy band diagram shows the bottom edge of conduction band, Ec , and top edge of valence band, Ev . · Ec and Ev are separated by the band gap energy, Eg .

  8. Conduction Band E c E Donor Level d Donor ionization energy Acceptor ionization energy Acceptor Level E a E v Valence Band Donor and Acceptor in the Band Model Ionization energy of selected donors and acceptors in silicon

  9. Device Fabrication Oxidation Lithography & Etching Ion Implantation Annealing & Diffusion

  10. Beginning from a silicon wafer Side View Top View

  11. Thermal Oxidation Side View Top View

  12. Spin-on Photo Resist (PR) Side View Top View

  13. Alignment, UV Expose and Develop Photo Resist (PR) Side View Top View

  14. Oxide Etched Side View Top View

  15. Remove Photo Resist (PR) Side View Top View

  16. Doping (implantation or diffusion) Side View Top View

  17. Grow Field Oxide (wet/dry) and dopant diffusion Side View Top View

  18. Spin-on Photo Resist (PR) Side View Top View

  19. Alignment, UV Expose and Develop Photo Resist (PR) Side View Top View

  20. Oxide Etched Side View Top View

  21. Remove Photo Resist (PR) Side View Top View

  22. Grow Gate Oxide (dry) Side View Top View

  23. Spin-on Photo Resist (PR) Side View Top View

  24. Alignment, UV Expose and Develop Photo Resist (PR) Side View Top View

  25. Field Oxide Etched Side View Top View

  26. Field Oxide Etched Side View Top View

  27. Metal (e.g., Aluminum) deposition Side View Top View

  28. Spin-on Photo Resist (PR) Side View Top View

  29. Alignment, UV Expose and Develop Photo Resist (PR) Side View Top View

  30. Aluminum Etched Side View Top View

  31. Remove Photo Resist (PR), annealing - complete Side View Top View

  32. – + V Donor ions I I N P N-type V P-type Forward bias Reverse bias PN Junction PN junction is present in perhaps every semiconductor device.

  33. N-region P-region Ef (a) Ec Ec Ef (b) Ev Ev Ec Ef (c) Ev Neutral Neutral Depletion layer P-region N-region Ec Ef (d) Ev Energy Band Diagram of a PN Junction Ef is constant at equilibrium Ec and Ev are known relative to Ef Ec and Ev are smooth, the exact shape to be determined. A depletion layer exists at the PN junction where n 0 and p 0.

  34. Ec Non-radiative recombination through traps Radiative recombination Ev • Light emitting diodes (LEDs) • LEDs are made of compound semiconductors such as InP and GaN. • Light is emitted when electron and hole undergoradiative recombination.

  35. LED Materials and Structure

  36. AlInGaP Quantun Well Common LEDs

  37. Forward biased V = 0 I V Reverse bias Forward bias Reverse biased SchottkyDiodes

  38. MOS: Metal-Oxide-Semiconductor Vg Vg gate gate metal SiO2 SiO2 N+ N+ P-body Si body MOS transistor MOS capacitor

  39. Gauss’s Law Surface Accumulation Vg <Vt

  40. Surface Depletion ( ) V > V g fb qV ox E c f q s gate E - + + + + + + f - - E - SiO v qV 2 - - - - - - - g - - - - - - - W - - - - - - - E E dep V c , f depletion layer depletion charge, Q region dep E v P-Si body M O S

  41. E c f st E i = f q A C B E f D B E = qV qV v g t E E c , f E v M O S Threshold Condition and Threshold Voltage Threshold (of inversion): ns = Na, or (Ec–Ef)surface= (Ef –Ev)bulk , or A=B, and C = D

  42. + for P-body, – for N-body Threshold Voltage

  43. > V V g t E c - E gate f - - E - v ++++++++++ - - SiO 2 qV g - - - - - - - - - - - - - - - V E E c , f Q Q dep inv E - Si substrate P v M O S Strong Inversion–Beyond Threshold Vg > Vt

  44. Basic MOSFET structure and IV characteristics + +

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