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. Outline
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 Carbon Nanotube Field Effect Transistor
(CNTFET)
 NEGF Formalism
 Results
 Quantum Effects
 I-V Characteristics
 Scaling Effects

3. Objective
 Analysis of ballistic transport in CNTFETs.
 Comparison of performance between
Schottky-Barrier & MOSFET-like
CNTFETs.
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4. Carbon Nanotube (CNT)
 Rolled up Graphene sheet
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A spinning Carbon
Nanotube

5. CNT Types
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(a) zigzag type
(b) armchair type

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.
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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
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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.
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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
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10. CNTFET Structures
 Back Gated CNTFETs
 Top Gated CNTFETs
 Vertical CNTFETs
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Back Gated CNTFET
Top Gated CNTFET Vertical CNTFET

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
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12. Schottky-Barrier CNTFET
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13. Doped Contact CNTFET
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14. NEGF Formalism Review
 Retarded Green’s
function in matrix form,
 Hamiltonian matrix
for the subbands,
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15. NEGF Formalism Review (contd.)
 Current,
 Where T(E) is
the transmision
coefficient,
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16. NEGF Formalism Review (contd.)
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Self-consistantly solving NEGF & Poisson’s Equation

17. Device Structure & Parameters
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 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. Results
 Quantum Effects
 Quantum-Mechanical Interference
 Quantum Confinement
 Tunneling
 I-V characteristics
 Effect of Gate Dielectric Constant
 Scaling Effects
 Diameter
 Length
 Oxide Thickness
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19. Quantum Effects
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Quantum-Mechanical Interference Quantum Confinement
At VGS = 0.5V and VD=0.5V for doped contact CNTFET

20. Quantum Effects (contd.)
Tunneling in Channel Region of
Schottky-Barrier CNTFET [1]
Current in Channel Region of
Doped Contact CNTFET
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[1] J. Guo, “Carbon Nanotube Electronics: Modeling, Physics and Applications”

21. I-V Characteristics
 ID-VD Comparison
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Schottky-Barrier CNTFET Doped Contact CNTFET
Doped Contact CNTFET provides more current for same VGS.
5 uA
15 uA

22. I-V Characteristics (contd.)
 ID-VGS Comparison
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Schottky-Barrier CNTFET Doped Contact CNTFET

23. Effect of Gate Dielectric Constant
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Schottky-Barrier CNTFET Doped Contact CNTFET [Table]
Constant table
Higher Dielectric Constant provides more Drain Current
2.5 uA
7.5 uA

24. Effect of Gate Dielectric Constant
(contd.)
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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. Scaling Effects: Diameter
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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. Scaling Effect: Channel Length
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Schottky-Barrier CNTFET Doped Contact CNTFET
[Table]
Channel Length have very negligible effect on Drain Current.

27. Scaling Effect: Length (contd.)
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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. Scaling Effect: Oxide Thickness
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Schottky-Barrier CNTFET Doped Contact CNTFET [Table]
Thinner oxide provides much more ON/OFF ratio for both types of CNTFETs.

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
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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. 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.
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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.
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32. Questions
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33. Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 33
Thank You

34. Dielectric Constant Table [3]
Oxide Material Dielectric Constant, k
SiO2 3.9
Si3N4 8
HfO2 14
ZrO2 25
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[3] Robertson, J. "High dielectric constant oxides." The European Physical Journal Applied Physics 28.03 (2004): 265-291.
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35. Simulator Software Screenshot
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CNTFET Lab Cylindrical CNT MOSFET Simulator

36. Effect of Diameter
 Bandgap,
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