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

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

3. AsB Dopants, Electrons and holes

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

5. GaAs, III-V Compound Semiconductors, and Their Dopants 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? GaAs Ga

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 thevalence band.  The lowest empty band is theconduction band.

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

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

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

10. Side ViewTop View Beginning from a silicon wafer

11. Side ViewTop View Thermal Oxidation

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

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

14. Side ViewTop View Oxide Etched

15. Side ViewTop View Remove Photo Resist (PR)

16. Side ViewTop View Doping (implantation or diffusion)

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

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

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

20. Side ViewTop View Oxide Etched

21. Side ViewTop View Remove Photo Resist (PR)

22. Side ViewTop View Grow Gate Oxide (dry)

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

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

25. Side ViewTop View Field Oxide Etched

26. Side ViewTop View Field Oxide Etched

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

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

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

30. Side ViewTop View Aluminum Etched

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

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

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

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

35. LED Materials and Structure

36. Common LEDs AlInGaP Quantun Well

37. V I Reverse bias Forward bias V = 0 Forward biased Reverse biased Schottky Diodes

38. MOS: Metal-Oxide-Semiconductor SiO 2 metal gate Si body VgVg gate P-body N+ N+ MOS capacitor MOS transistor VgVg SiO 2 N+ N+

39. Surface Accumulation Gauss’s Law V g <V t

40. Surface Depletion ( ) g V> V fb SiO 2 gate P-Si body + + + - - - - - - - V depletion layer charge,Q dep - - - - - - -

41. Threshold Condition and Threshold Voltage Threshold (of inversion): n s = N a, or (E c –E f ) surface = (E f – E v ) bulk, or  A=B, and C = D E c, E f M O S E v E f E i E c A B C = q   E v D qV g = t  st

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

43. Strong Inversion–Beyond Threshold V g > V t

44. Basic MOSFET structure and IV characteristics + +