graphene transistor

7/30/2019 Graphene Transistor

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"Great things are done by a series of small things brought together."

-- Vincent Van Gogh



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.



•

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?



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.



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.



a) Graphite films visualizedthrough atomic forcemicroscopy.

b) Transmission electronmicroscopy image

c) Scanning electron

microscope image of

graphene.

Samples of graphene



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.



Properties: charge carriers

• Samples are excellent- graphene is ambipolar: charge carrier

concentration continuously tunable from electrons to holes in high

concentrations



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.



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.



• 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.



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



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



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.



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



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.



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.



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.

graphene transistor

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