Carbon Nano Tube Transistor structure and design

1. Carbon Nanotube FET

2. Agenda

3. Introduction
CNTFETs
CNTFETs are a new kind of molecular device that using a CNTs as channel,
and are regarded as an important contending device to replace
conventional silicon transistors. First carbon nanotube was introduced at
1991 by Sumio Iijima from NEC. The first simple CNTFET, reported in
1998, was manufactured by depositing single-wall CNTs from solution
onto oxidized silicon wafers. They are can provide low-power with high
performance and high power density in smaller ICs.

4. What is CNT ?
• Large molecules of pure carbon that are long and thin
cylinders, about 0.4 to 2 nm in diameter, and 0.2nm
to 5m long.
• Structures:
• Single-walled
• Multi-walled
• Special Properties
• Nano rang dimension
• High mobility of carrier
• High temperature stability
• Ballistic carrier transition

5. Carbon Molecules
• Graphene is single layer of atoms arranged
in a two-dimensional.
• Graphite is multi layer of graphene, more
than of 10 layer.
• Diamond is converted form of graphite
under high pressures and temperatures.
• Fullerene is molecule consists of carbon so
as to form a closed mesh, like a ball.
• Carbon nanotube is cylindrical molecule,
consist of rolled-up sheets of graphene.

6. CNTs Structure
Single-Walled CNTs (SWCNT)
• A single layer of graphene wrapped into a cylindrical.
• Diameter 0.4~2nm and length about 0.2nm~5m
• Band gap range about 0 ~ 2 e. V from
• Conductivity has metallic or semiconductor properties
• Popular Used in Nano electronics applications like nano-
wire and CNTFETs
0.4
~
2
nm

7. CNTs Structure
Multi-Walled CNTs (MWCNTs)
• Consist of multiple rolled layers of graphene, or a
single sheet of graphene rolled around itself.
• Distance between layers > 3. 8 Angstroms
• Usually a zero-gap and metal Properties
• Good for Nano mechanical devices Triple-walled
armchair CNT
> 3.8 𝐴

8. Chirality of CNTs
• Hexagonal orientation and diameter of the
nanotubes determine its properties
• Chiral vector is defined as 𝐶ℎ = (𝑛𝑎1,𝑚𝑎2)
• Distance between atom is 0.142nm (1.42 𝐴°
)

9. Chirality of CNTs
• Zig-Zag (n,0)
• In 3n form of 𝑎1𝑣𝑒𝑐𝑡𝑜𝑟 take a Metallic and
another form take a semiconductor property
• Arm-chair (n=m)
• Always metallic properties
• Chiral (n , m)
• Conductive property depended of 𝐶ℎ

10. How are CNTs created ?
• Chemical Vapor Deposition (CVD)
• Achieved by taking a carbon source in a gas
phase and using an energy source, such as a
resistively heated coil, to transfer energy to a
gaseous carbon molecule. Hydrocarbons then
flow through a quartz tube at high temp to break
the HC bonds to produce pure Carbon.
• Arc Discharge
• Uses two graphite rods, one anode and one
cathode at 1mm apart, placed in an low pressure
inert gas. Most nanotubes deposit on cathode
when the rods electrodes kept at different
potentials and Arc.

11. CNTFET
The first CNTFET was fabricated in 1998.
Like MOSFETs, CNTFETs have three terminals: source,
gate and drain. When the gate is on, the current transmits
from the source to the drain through a semiconducting
CNTs channel. The segment between the drain/source
and the gate is heavily doped to provide low resistance.
CNTFETs have very promising I-V and transfer
characteristics and hyper mobility.

12. CNTFET TYPE
• Based on geometry
• Top Gate
• Bottom Gate
• Coaxial Gate
• Based on operation
• Schottky barrier
• Ohmic barrier like the MOSFET

13. CNTFET First Fabrication
• depositing single-wall CNTs that
synthesized by laser ablation, from
solution onto oxidized Si wafers
which had been prepatterned with
gold or platinum electrodes.

14. CNTFETs Geometric Type
BACK-GATE TOP-GATE WRAP-AROUND SUSPENDED GATE
• Basic generation shown High on-state resistance, low transconductance, no current saturation profile.
• Second generation of CNFET came in TOP-GATE structure to improve performance.
• Next generation is Wrap-AROUND to achieved more electrostatic control over channel.

15. SB and Ohmic Characteristics

16. CNTFETs Fabrication
SUBTITLE
• Popular way is high resolution Ion-Beam technology
• Deposition silicon oxide
• Lift-Off in metallization
• Passivation

17. IDS Characteristics
• D-S current determined by the Fermi-Dirac probability distributions:
𝑁𝑆 =
1
2
න 𝐷(𝐸)𝑓 𝐸 − 𝑈𝑆𝐹 𝑑𝐸
𝑁𝐷 =
1
2
න 𝐷(𝐸)𝑓 𝐸 − 𝑈𝐷𝐹 𝑑𝐸
𝑈𝑆𝐹 = 𝐸𝐹 − 𝑞𝑉𝑆𝐶
𝑈𝐷𝐹 = 𝐸𝐹 − 𝑞𝑉𝑆𝐶 − 𝑞𝑉𝐷𝑆
• D(E): density of states at the Channel
• Ns : velocity states filled by the Source
• Nd : velocity states filled by the Drain

18. I-V Characteristics
• Ambipolar in 𝑉𝐺𝑆 =
𝑉𝐷𝑆
2
• Ambipolar point independete to SB

19. 𝐼𝐷𝑆 Dependency
Dependency of Temperature
Dependency of diameter of CNT
• Increase current by increasing temperature
• Reduce current by increasing diameter of CNT channel

20. N-Type or P-Type CNTFETs
ATMOSPHERIC ADSORPTION
• Previously, all CNTFETs were p-type
because contact doping by the adsorption
of oxygen from the atmosphere was not
understood.
• Atmospheric oxygen near the metal and
nanotube contacts affects the bending of
the Fermi level and kept it near the valence
band, which makes injection of holes
easier.
OXYGEN DESORPTION
• Oxygen desorption at high temperature in
the vacuum, adapts the Fermi level near
the conduction band, allowing the injection
of electrons.
• using thermal annealing, there is no
threshold voltage shift when making n-
type from p-type .

21. Ambipolar CNTFETs
• A back gated n-type CNTFETs can be achieved by
doping the CNT with potassium.
• Shift the Fermi level from the valence band edge to
the conduction band edge by transferring the
electrons from potassium atoms to the CNTs.
• An intermediate state where both electrons and
holes are allowed to ambipolar conduction.

22. CNTFETs Features
COMPARISON TO MOSFET
• In comparison to MOSFET and Fin-FET
(90~7nm), SWCNTs have diameters is
0.4 to 5nm.
• Three times faster than silicon-based
transistors in a same power.
• Extremely high charge-carrier and
mobility higher than silicon by a factor
of 200.
• High current densities of up to 1010
A/cm 2 in comparted to the current
density of copper, that is 107
A/cm 2
• The carbon nanotubes can works as a
device to operate in the ballistic regime.
ADVANTAGE
• Better control over channel
• Better threshold voltage
• High electron mobility
• High current density
• High transconductance
• High linearity performance
• metal-nanotube contact Schottky
barrier represents the active
switching element
DISADVANTAGE
• degrade in a few days when
exposed to oxygen.
• Reliability
• Difficulties in mass production,
production cost

23. New Efforts
• INNER GATE TECHNOLOGY OF CNTFETS
• Improve electromagnetic control on channel
• Hi performance in switching technology
• Reduce mechanical effect on CNT channel

24. Summary CNTFETs are provide dense, high performance, and low power circuits. It is a rapidly developing
technology due to its outstanding electrical characteristics. They are the most promising alternative
for conventional transistors.
In theory, CNTFETs have the potential to reach the terahertz regime when compared to standard
semiconductor technologies. Nevertheless, this field is still at an early stage, and for the time being,
researchers should remain focused on lowering the process variation.
Technology advancements such as 5G networks increase the pressure to improve smartphone battery
life, spectral efficiency, and more. One potential solution is the use of carbon nanotube field-effect
transistors (CNTFETs).

25. Thank You

26. • REFERENCES
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