Carbon nanotubes (CNTs) as a main factor in state of the art transistor technologies. Due to constant miniaturization the SCR is reached to its bottleneck and we really need something in there that could reduce the scattering and pinching in channel.

1. Dresden, 23.09.2011
CNT Ballistic Transistor
Ballistic Carbon Nanotube Field-Effect Transistors
Ali Javey et al.
By
Nikita Konshin and Tashfain Yousuf
May 9th 2018
Subject: Nanostructured Materials
Prof. Dr. Larysa Baraban
TU Dresden

2. © Tashfain and Nikita TUD
CNT Ballistic Transistor Definition
Carbon structures
Angewandte Chemie. 55 (37): 10962–10976.
CNT: Carbon allotrope rolled graphene sheet.
Ballistic: Ballistic transport, having negligible
electrical resistivity caused by scattering.
FET transistor: Allow to control the behavior of
the device with electric field.
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3. © Tashfain and Nikita TUD
Challenge:
Presence of Schottky barrier at nanotube-metal
junction.
Problems:
Limitation of the transistor conductance and current
delivery.
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(Ph. Avouris, Acc. )
Challenge and Problem

4. © Tashfain and Nikita TUD
Advantages of Palladium (Pd) and CNT
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SEM Image of FET
(Javey, 2003)
Ballistic transport limit of 4*e2/h
High current-carrying
capability (0.25 mA per tube)

5. © Tashfain and Nikita TUD
SW-CNT FET
(Javey, 2003)
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• Fabry–Perot interferences at
low temperatures.
• Under high voltage
operation, the current
saturation appears to be set
by backscattering of the
charge carriers by optical
phonons.
• High-performance ballistic nanotube
field effect transistors with zero or
slightly negative Schottky barriers.

6. © Tashfain and Nikita TUD
CNT FET Operation
• Schottky barriers (SB) at contacts effects the
conductance rather than gate voltage.
• Thermal Equilibrium carriers move from Hi
concentration to Low.
• Probability of the barrier penetration exponentially
depends on the barrier height.
(Zeghbroeck, 2004)
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7. © Tashfain and Nikita TUD
Results and Discussion
Thermionic Emission and tunneling limits the
ON state conductance of nanotube FETs to be
well below the 4e2/h limit.
10–100 times more resistive than at room
temp.
To eliminate or greatly suppress SBs at metal–
nanotube contacts. We find that for Pd-
contacted:
• Long SWNT at Room Temp. VB Gon<0.1*(4e2/h)
• Short SWNT — VB Gon={0.4,0.5}* (4e2/h)
3um SWNT
300 nm SWNT
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8. © Tashfain and Nikita TUD
Schottky Barrier and Workfunction
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Ion
depends
on SB
SB
depends
on Metal
Workfuncti
on
Metal
Workfunction
is engineered
by absorption
of O2 and H2
Exposure of
Pd to
molecular
hydrogen
reduces its
work function
@ RT

9. © Tashfain and Nikita TUD
Experimental Results Pd to H2 Exposure
• Experiment results:
Higher SB for holes: decreased p-channel conductance.
Lower SB for electrons: increased n-channel conductance.
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10. © Tashfain and Nikita TUD
CNT FET and MOSFET
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CNT FET
(Bob Yirka, Phys.org)
MOSFET
(Intel 22nm FinFET)

11. © Tashfain and Nikita TUD
CNTFET Vs MOSFET
• Gate voltage controls the channel conduction in both FETs.
• CNT is used as electron transport channel in CNTFETs.
• MOSFETS has heavily doped Si drain and Source, while CNTFETs uses
metals.
• No doping is needed to create an n-type or p-type CNTFET.
• The type of CNT-FET depends on the band-gap of the CNT and the
workfunction of the metal used as the S/D contact.
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12. © Tashfain and Nikita TUD
IV Curve of N type CNT MOS
• IV Characteristics of p-
channel CNTFET corresponds
to PMOS FET.
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13. © Tashfain and Nikita TUD
Ballistic Transport Properties
• Depends on nanotube diameter and length.
• At Room Temperature.
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Diameter d Length L RON
1.7nm 275nm 32kΩ
<2nm 275nm constant
~ 300nm 10kΩ Ballistic

14. © Tashfain and Nikita TUD
Ion and IOFF data P-SWCNT FET
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Ion max IDsat<25umA
GOAL
IDsat

15. © Tashfain and Nikita TUD
Operation & Results of CNTFET
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Propertes Characteristics Data Subthreshold swing
ION&OFF
Short Tubes Ballistic 10kΩ S < 170 mV per
decade
Long Tubes Diffusive 37kΩ S < 150 mV per
decade
Large Thermal
Activation Barriers
Limits IOFF
High ON state
conductance GON
GON 0.65 x 4e2/h @ RT SB-free MOSFET

16. © Tashfain and Nikita TUD
Conclusion & Summary
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Output
Characteristics
• Model Vgs >= - 2.5 V
• Hole mobility of uh = 4,000
cm2/Vs
Saturation current
IDsat > 10 uA
• grows slower than expected
from square-law model as
more -Vgs is applied,
especially when IDsat
approaches 25 uA.
High Ion flows in CNTs
• Optical phonon
backscattering SPhonon(this
effect has not been
observed in earlier tube FETs
with maximum currents at 7
uA).
• Preliminary analysis based
on a ballistic MOSFET model
leads to good but non-ideal
fitting of the experimental
data.

17. © Tashfain and Nikita TUD
Conclusion & Summary
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• Below optical Sphonon MFP(100 nm) could lead
to ballistic transport in the high I regime.
• I delivery capability of nanotube FETs.
NT Channel
length scaling
• Fails to reproduce the device characteristics.
Diffusive
transport
model
• Theoretical work needed to model detailed
behavior of nanotube transistor.
Ballistic
transport
regime