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  • 1. Ballistic Transport in Schottky-Barrier and MOSFET-like Carbon Nanotube Field Effect Transistors: Modeling, Simulation and Analysis Presented by: Protik Das Exam Roll: 2240Department of Applied Physics, Electronics & Communication Engineering, University of Dhaka 1
  • 2. Outline Carbon Nanotube Field Effect Transistor (CNTFET) NEGF Formalism Results  Quantum Effects  I-V Characteristics  Scaling EffectsDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 2
  • 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. Carbon Nanotube (CNT) Rolled up Graphene sheet A spinning Carbon NanotubeDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 4
  • 5. CNT Types(a) zigzag type(b) armchair typeDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 5
  • 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 structureShowed in figure.Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 6
  • 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 Newtons 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 >> LDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 7
  • 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. 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 electrostaticsDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 9
  • 10. CNTFET Structures Back Gated CNTFETs Top Gated CNTFETs Vertical CNTFETs Back Gated CNTFET Top Gated CNTFET Vertical CNTFETDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 10
  • 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 voltageDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 11
  • 12. Schottky-Barrier CNTFETDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 12
  • 13. Doped Contact CNTFETDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 13
  • 14. NEGF Formalism Review Retarded Green’sfunction in matrix form, Hamiltonian matrixfor the subbands,Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 14
  • 15. NEGF Formalism Review (contd.) Current, Where T(E) isthe transmisioncoefficient,Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 15
  • 16. NEGF Formalism Review (contd.) Self-consistantly solving NEGF & Poisson’s EquationDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 16
  • 17. Device Structure & Parameters 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 0.68eVDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 17
  • 18. Results Quantum Effects  Quantum-Mechanical Interference  Quantum Confinement  Tunneling I-V characteristics Effect of Gate Dielectric Constant Scaling Effects  Diameter  Length  Oxide ThicknessDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 18
  • 19. Quantum EffectsQuantum-Mechanical Interference Quantum Confinement At VGS = 0.5V and VD=0.5V for doped contact CNTFETDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 19
  • 20. Quantum Effects (contd.)Tunneling in Channel Region of Current in Channel Region ofSchottky-Barrier CNTFET [1] Doped Contact CNTFET[1] J. Guo, “Carbon Nanotube Electronics: Modeling, Physics and Applications”Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 20
  • 21. I-V Characteristics ID-VD Comparison Doped Contact CNTFET provides more current for same VGS. 15 uA 5 uA Schottky-Barrier CNTFET Doped Contact CNTFETDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 21
  • 22. I-V Characteristics (contd.) ID-VGS Comparison Schottky-Barrier CNTFET Doped Contact CNTFETDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 22
  • 23. Effect of Gate Dielectric Constant Higher Dielectric Constant provides more Drain Current 7.5 uA 2.5 uA Schottky-Barrier CNTFET Doped Contact CNTFET [Table]Constant table Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 23
  • 24. Effect of Gate Dielectric Constant (contd.) K = 3.9 K = 14The conduction band profile of SB CNTFETat VG= 0.5V . The solid line is for k = 25 thedashed 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 Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 24
  • 25. Scaling Effects: Diameter Lower diameter provides better ON/OFF ratio.ID− VGS characteristics at VD= 0.5V for SB ID− VGS characteristics at VD= 0.5VCNTFET. The solid line with circles is for for doped contact CNTFET.d ∼1nm, the sold line is for d ∼1.3nm,and the dashed line is for d ∼2nm [3] [3] J. Guo, “Carbon Nanotube Electronics: Modeling, Physics and Applications” [Table] [Cause] Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 25
  • 26. Scaling Effect: Channel LengthChannel Length have very negligible effect on Drain Current. Schottky-Barrier CNTFET Doped Contact CNTFET [Table]Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 26
  • 27. Scaling Effect: Length (contd.) Lch = 30nm Lch = 15nm Lch = 5nm Conduction band profile for doped contact CNTFET at (a) Lch= 30mn, (b) Lch = 15nm & (c) Lch = 5nm for VGS= 0.5V and VDS= 0.3VDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 27
  • 28. Scaling Effect: Oxide ThicknessThinner oxide provides much more ON/OFF ratio for both types of CNTFETs. Schottky-Barrier CNTFET Doped Contact CNTFET [Table] Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 28
  • 29. Overview of Our Findings Parameter Effect Comment Dielectric Constant, k Higher k provides better Doped Contact CNTFET electrostatic control gives better performance Channel Diameter Lower diameter provides Doped Contact have higher current higher ON/OFF ratio Channel Length Channel length have No mentionable negligible effect on I-V advantage for length Oxide Thickness Thinner oxide provides Doped Contact CNTFET much higher ON/OFF ratio have higher ratio than SBOne of our key findings: Thinner oxide provides much higher ON/OFF ratio butit also increases leakage current. So using thinner oxide of higher k ensures lessleakage current & gives more electrostatic control over channel. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 29
  • 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. 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. QuestionsDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 32
  • 33. Thank YouDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 33
  • 34. Dielectric Constant Table [3] Oxide Material Dielectric Constant, k SiO2 3.9 Si3N4 8 HfO2 14 ZrO2 25[3] Robertson, J. "High dielectric constant oxides." The European Physical Journal Applied Physics 28.03 (2004): 265-291. returnDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 34
  • 35. Simulator Software Screenshot CNTFET Lab Cylindrical CNT MOSFET SimulatorDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 35
  • 36. Effect of Diameter Bandgap, returnDepartment of Applied Physics, Electronics & Communication Engg., University of Dhaka 36