Transcript LEGO Mindstorm Robots Implementing Tic Tac Toe

Carbon Nanotube
Transistors
By Paul Baumstarck,
Miguel Taveras,
and Andrew White
1
Overview
Background
Major contributors
Benefits of CNFETs
Difficulties and problems with CNTs
The future of carbon CNTs
2
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.
3
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.
4
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.
5
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
6
Three Basic CNFET Designs
Two common designs:

Coaxial Shottky-Barrier
CNFETs

MOSFET-like CNFETs
Ambipolar I-V Behavior
7
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
8
Single-Electron CN Transistor
Bending of a Single Walled
Carbon Nanotube
Researches at Delft University in the
Netherlands created the world’s first singleelectron 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.
SET and “Conducting Island”
9
Circuit Example: CNFET Inverter
CNFETs have already been used to implement
basic logic circuits such as the inverter.
CNFET Inverter and Voltage
Transfer Characteristic
10
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
11
Major Contributors: NEC laboratory
One of the biggest and longeststanding 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.
12
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
13
Major Contributors: IBM (Cont.)
Constructive Deconstruction:
Const. Deconst. Process Diagram
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.
14
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
D. L. John, Leonardo C.
Castro, Jason Clifford, and
David L. Pulfrey at the
University of British
Columbia, Vancouver,
Canada
15
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
16
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.
17
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.
18
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
19
Problem Statement
We examined how the tunneling probability in SBCNFETs depends on the diameter of the nanotube and
possible applications of this to electronic circuit design.
Expected Energy Band Diagram for SBCNFET with Applied Gate Voltage
SB-CNFET Diagram
20
Application Problem (Cont.)
Assume triangular tunneling barrier as in
textbook:
Triangular Barrier Tunneling Equations
21
Application Problem (Cont.)
Results of tunneling probability
calculations:
Tunneling Probability vs.
Energy with Varying Diameter
3-D Tunneling Probability vs.
Energy and Diameter
22
Application Problem (Cont.)
Circuit applications: multiple-valued logic
Ternary Inverter Implemented
with CN-FETs
Power Delay Product
Comparisons for CN-FET
Ternary Logic
23
Carbon Nanotube
Devices
By Paul Baumstarck,
Miguel Taveras,
and Andrew White
24