Potential of AI (Generative AI) in Business: Learnings and Insights
My project
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:
Abdullah Al Mamun
Exam Roll: 2233
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. 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
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 4
A spinning Carbon
Nanotube
5. CNT Types
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 5
(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.
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 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. 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 electrostatics
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 9
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. 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
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. NEGF Formalism Review (contd.)
Current,
Where T(E) is
the transmision
coefficient,
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 15
16. NEGF Formalism Review (contd.)
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 16
Self-consistantly solving NEGF & Poisson’s Equation
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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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
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
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. Simulator Software Screenshot
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 35
CNTFET Lab Cylindrical CNT MOSFET Simulator
36. Effect of Diameter
Bandgap,
Department of Applied Physics, Electronics & Communication Engg., University of Dhaka 36
return