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Department of Applied Physics, Electronics & Communication Engineering, University of Dhaka 1
Ballistic Transport in Schottky-Barrier andMOSFET-like Carbon Nanotube Field EffectTransistors: Modeling, Simulation and AnalysisPresented by:
Abdullah Al MamunExam Roll: 2233
Outline
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Carbon Nanotube Field Effect Transistor (CNTFET)
NEGF Formalism Results
Quantum Effects I-V Characteristics Scaling Effects
Objective
Analysis of ballistic transport in CNTFETs. Comparison of performance between
Schottky-Barrier & MOSFET-like CNTFETs.
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Carbon Nanotube (CNT)
Rolled up Graphene sheet
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A spinning Carbon Nanotube
CNT Types
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(a) zigzag type
(b) armchair type
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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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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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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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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CNTFET Structures
Back Gated CNTFETs Top Gated CNTFETs Vertical CNTFETs
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Back Gated CNTFET
Top Gated CNTFET Vertical CNTFET
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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Schottky-Barrier CNTFET
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Doped Contact CNTFET
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NEGF Formalism Review
Retarded Green’s
function in matrix form,
Hamiltonian matrix
for the subbands,
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NEGF Formalism Review (contd.) Current,
Where T(E) is
the transmision
coefficient,
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NEGF Formalism Review (contd.)
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Self-consistantly solving NEGF & Poisson’s Equation
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
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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Quantum Effects
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Quantum-Mechanical Interference Quantum Confinement
At VGS = 0.5V and VD=0.5V for doped contact CNTFET
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”
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 uA15 uA
I-V Characteristics (contd.)
ID-VGS Comparison
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Schottky-Barrier CNTFET Doped Contact CNTFET
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
Effect of Gate Dielectric Constant (contd.)
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The 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
K = 3.9
K = 14
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]
Scaling Effect: Channel Length
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Schottky-Barrier CNTFET Doped Contact CNTFET
[Table]
Channel Length have very negligible effect on Drain Current.
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
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.
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.
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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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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Questions
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Thank You
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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Simulator Software Screenshot
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CNTFET Lab Cylindrical CNT MOSFET Simulator
Effect of Diameter
Bandgap,
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