1. Carbon Nanotube Transistors
By Paul Baumstarck,
Miguel Taveras,
and Andrew White

2. Overview Background Major contributors Benefits of CNFETs
Difficulties and problems with CNTs
The future of carbon CNTs

3. What is a carbon nanotube?
A carbon nanotube is a graphene sheet (with carbon atoms appearing in a hexagonal pattern) rolled up to form a hollow cylinder. CNTs have extremely low electrical resistance because electrons can travel for large distances without scattering (ballistic transport). This is partly due to their very small diameter and huge ratio of length to diameter. Also, because of their low resistance, CNTs dissipate very little energy. This will prove useful in solving the power consumption problems that are plaguing Silicon circuits.

4. Carbon nanotube formation
Graphene Sheet
Semiconductor Types
E-k Relations
The vector C in the figure above is the vector normal to the circumference vector in the direction the tube is rolled.
C = na1 + ma2 gives the vector C. The values for the scalars
n and m determine whether the nanotube is conducting or
semi-conducting.

5. MOSFETs: The End of an Age
Past 40 years MOSFETs have been most widely used technology.
Latest technology MOSFET has minimum feature of 0.25 micron.
Quantum mechanics and fabrication techniques are limiting minimum feature size.
New nano-devices will take advantage of quantum mechanical phenomena which was previously ignored.

6. Carbon Nanotube Field Effect Transistor
In the generic CNFET a carbon nanotube is placed between two electrodes while a separate gate electrode controls the flow of current in the channel.
Basic CNFET Design
Actual CNFET

7. Three Basic CNFET Designs
Two common designs:
Coaxial Shottky-Barrier CNFETs
MOSFET-like CNFETs
Ambipolar I-V Behavior

8. P- and N-Type CNFET I-V Plots
P- and N-Type CNTs
CNTs are naturally p-type because exposure to the oxygen in the air causes the Fermi level to shift towards the valence band. However, there are several methods available to convert p-type CNTs to n-type:
Annealing in vacuum or H2
Doping with Potassium
P- and N-Type CNFET I-V Plots

9. Single-Electron CN Transistor
Researches at Delft University in the Netherlands created the world’s first single-electron transistor by placing two sharp bends (i.e., large potential barriers) in a CNT 20 nm apart to create a “conducting island” that electrons must tunnel in to.
Bending of a Single Walled Carbon Nanotube
SET and “Conducting Island”

10. Circuit Example: CNFET Inverter
CNFETs have already been used to implement basic logic circuits such as the inverter.
CNFET Inverter and Voltage Transfer Characteristic

11. Major Contributors: Sumio Iijima
A leading researcher at NEC Laboratories since the late 1980’s.
Discovered carbon nanotubes in 1991 by demonstrating that the carbon fibers produced by a carbon arc were hollow.
Also showed how to produce “nanohorns” at room temperature and with high yield (nanohorns had been previously discovered by Peter Harris). Nanohorns have since proven useful in next-generation fuel cells because they have very high absorbability.
Nanohorn
Nanohorn Bundle

12. Major Contributors: NEC laboratory
One of the biggest and longest- standing corporation laboratories in nanotube research.
Have used nanohorns to produce small fuel cells for mobile applications.
Are active in CNFET design:
They have developed a stable fabrication technology for CNT transistors, and transistors produced with this process have demonstrated more than 10 times the transconductance of silicon MOS transistors.
One of NEC’s future research goals is to replace the bulky metal electrodes of their CNFETs with other nanotubes such as Boron-Nitride.

13. Major Contributors: IBM
IBM has used CNTs and their ambipolar characteristics to produce nanotube light sources. One prototype used a 1.4-nm diameter nanotube to produce light through the collision of holes and electrons. Varying the gate voltage also controlled where along the length of the CNT the light was emitted.
Nanotube Light
Varying the Light Emission Point

14. Major Contributors: IBM (Cont.)
Const. Deconst. Process Diagram
Constructive Deconstruction:
IBM developed a method for controlling which kinds of nanotubes are deposited on the substrate. Originally, both metallic and semiconducting CNTs are grown, then electrodes are placed across the bundles. A high voltage is applied across the electrodes which destroys the metallic CNTs but leaves the semiconducting ones intact.

15. Major Contributors: UBC
This group of researches has written many influential papers on the characteristics of SB-CNFETs including “Electrostatics of Coaxial Shottky-Barrier Carbon Nanotube Field-Effect Transistors” and “Electrostatics of partially gated carbon nanotube FETs.”
SB-CNFET Energy Band Diagrams
While Sumio Iijima and his group gave nanohorns their name, credit for their discovery actually belongs to Peter Harris and his colleagues.
D. L. John, Leonardo C. Castro, Jason Clifford, and David L. Pulfrey at the University of British Columbia, Vancouver, Canada

16. Benefits of CNFETs High single crystallinity
 Low defect density, grain boundary free
Predictable electron transport properties
 Reliable device performance
Unique properties due to quantum confinement effects
 Enhancement in device characteristics
Potential to revolutionize nano-scale science and technology

17. Advantages of CNTs over Silicon
As Silicon transistors are scaled down the doping of the channel has to increase proportionately while its volume decreases. The change in the number of dopants produces important differences in switching properties and degrades the overall performance of the system.
Nanotube transistors can operate even without dopants and are less sensitive to differences in the channel length. Instead, CNFETs depend on the diameter of the tube and its chirality.

18. CNT Challenges
The production methods available for CNTs either produce CNTs with widely varying sizes and chiralities or are prohibitively expensive.
Exposure to open air can cause an n-type CNT to revert back to p-type.
Placing CNTs on substrate is also a big challenge. Some prospective solutions are DNA Self-Assembly and using a electric field to direct CNT growth during Chemical Vapor Deposition.
The main obstacle to CNTs replacing Silicon transistors is that there are no mass production methods available for CNTs to rival the well-developed Silicon and photolithography process at present.

19. The Future Medium term (5-10 years) - Memory devices
- Fuel cells, batteries
- Biosensors (CNT, molecular)
- Biomedical devices
- Advances in gene sequencing
Long term (> 15 years)
- Nanoelectronics (CNT)
- Molecular electronics
- Use in new aerospace and automotive industry
composites

20. Expected Energy Band Diagram for SB-CNFET with Applied Gate Voltage
Problem Statement
We examined how the tunneling probability in SB-CNFETs depends on the diameter of the nanotube and possible applications of this to electronic circuit design.
Expected Energy Band Diagram for SB-CNFET with Applied Gate Voltage
SB-CNFET Diagram

21. Application Problem (Cont.)
Assume triangular tunneling barrier as in textbook:
Triangular Barrier Tunneling Equations

22. Application Problem (Cont.)
Results of tunneling probability calculations:
Tunneling Probability vs. Energy with Varying Diameter
3-D Tunneling Probability vs. Energy and Diameter

23. Application Problem (Cont.)
Circuit applications: multiple-valued logic
Ternary Inverter Implemented with CN-FETs
Power Delay Product Comparisons for CN-FET Ternary Logic

24. Carbon Nanotube Devices
By Paul Baumstarck,
Miguel Taveras,
and Andrew White