graphene transistor
TRANSCRIPT
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"Great things are done by a series of small things brought together."
-- Vincent Van Gogh
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ABSTRACT
• In modern age Engineers have paved the way for a new
generation of faster, more powerful cell phones, computers
and other electronics by developing a practical technique to
replace silicon with carbon on large surface.
• The material called “ Graphene” which is a single layer of
atoms arranged in honeycomb lattice could let electronics to
process information and produce radio transmission many
times better than silicon based devices such as transistors.
• Graphene transistors could scale to transistor channels as
small as two manometers (nm) with terahertz speeds.
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•
2-dimensional hexagonallattice of carbon
• sp2 hybridized carbon
atoms
• Basis for C-60 (bucky
balls), nanotubes, and
graphite
• Among strongest bonds in
nature
What is Graphene?
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A Two dimensional crystal
• In the 1930s, Landau and Peierls (and Mermin, later)showed thermodynamics
prevented 2-d crystals in free state.
• Melting temperature of thin films decreases rapidly with temperature ->monolayers generally unstable.
• In 2004, experimental discovery of graphene- high quality 2-d crystals
• Possibly, 3-d rippling stabilizes crystal
Representation
of rippling in
graphene. Red
arrows are
~800nm long.
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How to make graphene
• Strangely cheap and easy.
• Either draw with a piece of graphite, or repeatedly peel with
Scotch tape
• Place samples on specific thickness of Silicon wafer. Thewrong thickness of silicon leaves graphene invisible.
• Graphene visible through feeble interference effect.Different thicknesses are different colors.
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a) Graphite films visualizedthrough atomic forcemicroscopy.
b) Transmission electronmicroscopy image
c) Scanning electron
microscope image of
graphene.
Samples of graphene
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Electrons in Graphene
• Electrons in p-orbitals above andbelow plane
• p-orbitals become conjugatedacross the plane
• Electrons free to move acrossplane in delocalized orbitals
• Extremely high tensile strength -Graphene and graphite are great
conductors along the planes.
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Properties: charge carriers
• Samples are excellent- graphene is ambipolar: charge carrier
concentration continuously tunable from electrons to holes in high
concentrations
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Relativistic charge carriers
• Linear dispersion relation- charge carriers
behave like massless Dirac fermions with an
effective speed of light c*~106. (But cyclotron
mass is nonzero.)
• Relativistic behavior comes from interaction
with lattice potential of graphene, not from
carriers moving near speed of light.
•
• Behavior ONLY present in monolayer graphene;
disappears with 2 or more layers.
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Anomalous quantum Hall effect
• Classical quantum Hall effect.
– Apply B field and current. Charges build up on opposite sides of sample
parallel to current.
– Measure voltage: + and - carriers create opposite Hall voltages.
• Quantum Hall effect
– Classical Hall effect with voltage differences = integer times e2/h
• Fractional Quantum Hall effectQuantum Hall effect times rationalfractions. Not completely understood.
• Graphene shows integer QHEshifted by 1/2 integer
• Non-zero conductivity as charge
carrier density -> zero.
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• Hall conductivity
xy (red) and
resistivity xy vs.
carrierconcentration.
• Inset: xy in 2-
layer graphite.
• Half-integer QHE
unique to
monolayer.
*Note non-zero conductivity as carrier concentrations approach zero.
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Graphene Transistor
• Transistors less than one-
quarter the size of the
tiniest silicon ones - and
potentially more efficient -can be made using sheets of
carbon just one-tenth of a
nanometer thick
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Operation of Graphene Transistors at GHz
Frequencies
•
Top-gated graphene transistorsoperating at high frequencies (GHz)
have been fabricated. The work
represents a significant step towards
the realization of graphene-based
electronics for high-frequency
applications.Fig. A Optical image of the device layout
Fig. C Schematic cross-section of the graphene transistor Fig. B
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Graphene Nano ribbon Field-Effect Transistors
• Sub-10nm wide graphene
Nano ribbon field-effect
transistors (GNRFETs) are
studied systematically. Allsub-10nm GNRs afforded
semiconducting FETs
without exception, with
Ion/Ioff ratio up to 106 and
on-state current density as
high as ~2000μA/μm.
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Carbon Nanotube FETs
• Many different designs
– Carbon nanotube ring
• Semiconducting characteristics
•Conducting characteristics
– Carbon nanotube cantilever
• Single walled nanotube structure
(SWNT)• 2 separate designs using a metallic
multi-walled nanotube structure
(MWNT) acting as gate
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Materials and Substrate Preparation
• Graphitic films on SiC substrates
were prepared by solid-state
decomposition of single crystal
4HSiC (0001) in vacuum. The
method involves an inductively
heated vacuum furnace in which 3.5
mm X 4.5 mm X 0.3 mm SiC chips,
are heated to about 1400 °C.
• A typical SiC wafer will have a
Si-face in the front with a
[0001] normal, and a C-face in
the back with a [000-1] normal.
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Possible Applications
• High carrier mobility even at highest electric-field-inducedconcentrations, largely unaffected by doping= ballisticelectron transport over < m distances at 300K
– May lead to ballistic room-temperature transistors.
– GaTech group made proof of concept transistor- leaks electrons, butit’s a start.
• Energy gap controlled by width of graphene strip.
– Must be only 10s of nm wide for reasonable gap.
– Etching still difficult consistently and random edge configurationcauses scattering.
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Conclusion
• Although promising, graphene based electronics faces manyobstacles before it can become a competitive technology.
• Minimum conduction has to be decreased, device to device
variability has to be controlled, and a stable gate dielectric
must be found.
• However the chip level integration of hundreds of graphene
devices on insulating SiC substrates is a step towards making
graphene technology possible.
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