My project

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My project

  1. 1. Department of Applied Physics, Electronics & Communication Engineering, University of Dhaka 1 Ballistic Transport in Schottky-Barrier and MOSFET-like Carbon Nanotube Field Effect Transistors: Modeling, Simulation and Analysis Presented by: Exam Roll: 2233 & 2240
  2. 2. Outline Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 2  Carbon Nanotube Field Effect Transistor (CNTFET)  NEGF Formalism  Results  Quantum Effects  I-V Characteristics  Scaling Effects
  3. 3. Objective  Analysis of ballistic transport in CNTFETs.  Comparison of performance between Schottky-Barrier & MOSFET-like CNTFETs. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 3
  4. 4. Carbon Nanotube (CNT)  Rolled up Graphene sheet Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 4 A spinning Carbon Nanotube
  5. 5. CNT Types Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 5 (a) zigzag type (b) armchair type
  6. 6. Field Effect Transistor (FET)  The Field-Effect Transistor (FET) is a transistor that uses an electric field to control the conductivity of a channel in a semiconductor material. A generic FET structure Showed in figure. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 6
  7. 7. Keyword: Ballistic Transport  Ballistic Transport is the transport of electrons in a medium with negligible electrical resistivity due to scattering. Without scattering, electrons simply obey Newton's second law of motion at non-relativistic speeds.  Simply, Ballistic Transport is the transport of electrons in a channel considering no impurity or scatterer in the region.  Ballistic Transport can be considered when mean free path of an electron is greater than channel length. i. e., λ >> L Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 7
  8. 8. Carbon Nanotube FET (CNTFET)  A Carbon Nanotube Field Effect Transistor (CNTFET) refers to a field effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 8
  9. 9. Why Carbon Nanotube?  Near ballistic transport  Symmetric conduction/valence bands  Direct bandgap  Small size  Confinement of charge inside the nanotube allows ideal control of the electrostatics Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 9
  10. 10. CNTFET Structures  Back Gated CNTFETs  Top Gated CNTFETs  Vertical CNTFETs Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 10 Back Gated CNTFET Top Gated CNTFET Vertical CNTFET
  11. 11. CNTFET Operation  Schottky-Barrier CNTFET  Schottky-Barrier is formed between Source/Drain and channel  Direct tunneling through the Schottky barrier at the source- channel junction  Barrier width is controlled by Gate voltage  MOSFET-like/Doped Contact CNTFET  Heavily doped Source and Drain instead of metal  Barrier height is controlled by gate voltage Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 11
  12. 12. Schottky-Barrier CNTFET Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 12
  13. 13. Doped Contact CNTFET Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 13
  14. 14. NEGF Formalism Review  Retarded Green’s function in matrix form,  Hamiltonian matrix for the subbands, Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 14
  15. 15. NEGF Formalism Review (contd.)  Current,  Where T(E) is the transmision coefficient, Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 15
  16. 16. NEGF Formalism Review (contd.) Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 16 Self-consistantly solving NEGF & Poisson’s Equation
  17. 17. Device Structure & Parameters Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 17  Channel length, Lch = 20nm  Source/Drain length, LSD = 30nm  Oxide Thickness, tOX = 2nm  Dielectric Constant, k = 16  Source/Drain Doping, NSD = 1.5/nm  CNT (13, 0) diameter, 1.01nm  Bandgap 0.68eV
  18. 18. Results  Quantum Effects  Quantum-Mechanical Interference  Quantum Confinement  Tunneling  I-V characteristics  Effect of Gate Dielectric Constant  Scaling Effects  Diameter  Length  Oxide Thickness Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 18
  19. 19. Quantum Effects Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 19 Quantum-Mechanical Interference Quantum Confinement At VGS = 0.5V and VD=0.5V for doped contact CNTFET
  20. 20. Quantum Effects (contd.) Tunneling in Channel Region of Schottky-Barrier CNTFET [1] Current in Channel Region of Doped Contact CNTFET Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 20 [1] J. Guo, “Carbon Nanotube Electronics: Modeling, Physics and Applications”
  21. 21. I-V Characteristics  ID-VD Comparison Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 21 Schottky-Barrier CNTFET Doped Contact CNTFET Doped Contact CNTFET provides more current for same VGS. 5 uA 15 uA
  22. 22. I-V Characteristics (contd.)  ID-VGS Comparison Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 22 Schottky-Barrier CNTFET Doped Contact CNTFET
  23. 23. Effect of Gate Dielectric Constant Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 23 Schottky-Barrier CNTFET Doped Contact CNTFET [Table] Constant table Higher Dielectric Constant provides more Drain Current 2.5 uA 7.5 uA
  24. 24. Effect of Gate Dielectric Constant (contd.) Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 24 The conduction band profile of SB CNTFET at VG= 0.5V . The solid line is for k = 25 the dashed line for k = 8 and the dash-dot line for k = 1 [2] [2] J. Guo, “Carbon Nanotube Electronics: Modeling, Physics and Applications” Constant table K = 3.9 K = 14
  25. 25. Scaling Effects: Diameter Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 25 ID− VGS characteristics at VD= 0.5V for SB CNTFET. The solid line with circles is for d 1nm, the sold line is for d 1.3nm, and∼ ∼ the dashed line is for d 2nm [3]∼ ID− VGS characteristics at VD= 0.5V for doped contact CNTFET. [3] J. Guo, “Carbon Nanotube Electronics: Modeling, Physics and Applications” [Table] Lower diameter provides better ON/OFF ratio. [Cause]
  26. 26. Scaling Effect: Channel Length Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 26 Schottky-Barrier CNTFET Doped Contact CNTFET [Table] Channel Length have very negligible effect on Drain Current.
  27. 27. Scaling Effect: Length (contd.) Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 27 Conduction band profile for doped contact CNTFET at (a) Lch= 30mn, (b) Lch = 15nm & (c) Lch = 5nm for VGS= 0.5V and VDS= 0.3V Lch = 15nmLch = 30nm Lch = 5nm
  28. 28. Scaling Effect: Oxide Thickness Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 28 Schottky-Barrier CNTFET Doped Contact CNTFET [Table] Thinner oxide provides much more ON/OFF ratio for both types of CNTFETs.
  29. 29. Overview of Our Findings Parameter Effect Comment Dielectric Constant, k Higher k provides better electrostatic control Doped Contact CNTFET gives better performance Channel Diameter Lower diameter provides higher current Doped Contact have higher ON/OFF ratio Channel Length Channel length have negligible effect on I-V No mentionable advantage for length Oxide Thickness Thinner oxide provides much higher ON/OFF ratio Doped Contact CNTFET have higher ratio than SB Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 29 One of our key findings: Thinner oxide provides much higher ON/OFF ratio but it also increases leakage current. So using thinner oxide of higher k ensures less leakage current & gives more electrostatic control over channel.
  30. 30. Conclusions  The ON/OFF current ratio improves with high-κ gate dielectric.  This improvement is relatively higher in doped contact devices.  Thinner oxide provides better electrostatic control and improves device performance for both type of contacts. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 30
  31. 31. Future Perspectives  Completion of the partial code we have developed.  Convert the devices characteristic into SPICE model for circuit design.  Including the effect of phonon scattering. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 31
  32. 32. Questions Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 32
  33. 33. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 33 Thank You
  34. 34. Dielectric Constant Table [3] Oxide Material Dielectric Constant, k SiO2 3.9 Si3N4 8 HfO2 14 ZrO2 25 Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 34 [3] Robertson, J. "High dielectric constant oxides." The European Physical Journal Applied Physics 28.03 (2004): 265-291. return
  35. 35. Simulator Software Screenshot Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 35 CNTFET Lab Cylindrical CNT MOSFET Simulator
  36. 36. Effect of Diameter  Bandgap, Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 36 return

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