Transcript Tutorial 8 (PowerPoint)

Tutorial 8
Derek Wright
Wednesday, March 9th, 2005
Logic Devices
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Ferroelectric FETs
Resonant Tunneling Quantum Devices
Single-Electron Devices
Carbon Nanotubes
FeFETs
• Structure similar to a MOSFET
– Substrate with source/drain, dielectric, gate
• Dielectric has magnetic dipoles
• VGS can “flip” the dipole moment
• The dipole is either pointing towards or away
from the substrate
• One direction creates a channel of minority
carriers (inversion  ON)
• One direction pulls majority carriers towards the
gate (accumulation  OFF)
FeFET Operation
• Structure shows
hysteresis
• State stores as
which side of
hystersis curve
FET is on
• Must be
programmed
on/off
FeFET Structures
FeRAMs
• Nonvolatile RAMs can be made
that use FeFETs and Fe
capacitors
• a) DRAM
• b) FeRAM using Fe capacitor
• c) FeRAM using FeFET
a)
b)
c)
FeRAMs
• For smaller cell,
instead of 1T1C, fold
ferroelectric capacitor
into gate dielectric
• Challenge is dielectric
to silicon interface
– Buffer layer required
 series capacitance
FeRAMs
• By using High-k dielectric (LaAlO3), series
capacitance issue is reduced
• New stack shows good memory window
FeRAMs
• With the improved stack, good storage
characteristics are observed
Resonant Tunneling Quantum
Devices
• When structures are on the order of the
wavelength of an electron, quantum
effects become important
• Tunneling is one effect that is useful
• Since electrons are waves, they can have
resonance properties, too
• We can use resonance and tunneling
together to make devices with interesting
transfer characteristics
Resonant Tunneling
• Thin barriers allow tunneling
• However, the distance between two barriers
limits the electron’s energy to discrete values
• This results in discrete electron energies (lower
than the barrier) being allowed to pass
• It also distorts the transmission of energies
higher than the barrier due to interference
effects
Resonant Tunneling
Single Electron Devices
• Single electron devices:
– Benefit from scaling
– Dramatically reduce power
• Simple device has:
– a quantum dot
– a capacitively coupled gate
– a tunnel barrier
• Gate draws in or pushes
out an e- through the
tunnel barrier on the other
side
Single Electron Devices
• More than one
electron can enter the
box under discrete
gate bias
– Can accurately control
the number of
electrons in the dot
Single Electron Transistor
Single Electron Transistor
Single Electron Transistor
• Compared to MOSFETs, SETs:
– Consume less power
– Are more easily scalable
– Are easier to operate at low temperatures
– Must have a smaller source-drain voltage
What is A Carbon Nanotube?
• A cylinder of graphite (carbon)
• Capped by hemispherical ends
• Composed of pentagons and
hexagons
• Diameter from 0.5 – 2.0 nm
• Discovered by Sumio Ijyma
Single- and Multi-Wall Nanotubes
• MWNT is made
from layers of
SWNTs
• MWNTs can
have a diameter
of tens of nm
• Length can be
micrometers
SWNT
MWNT
Mechanical Properties of CNTs
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100x stronger than steel but 6x lighter
Highly flexible, unlike carbon fibers
Expansion when in E-field
High thermal resistance
Physical Properties of CNTs
• High surface area: 100s of m2/g
• Hollow CNTs enable molecule storage
inside
• Chemical treatment of CNTs allows other
molecules to be fixed to the surface
Electrical Properties of CNTs
• Metallic or semiconductor behavior based
on chirality
• Can be more conductive than copper
– Mobility  = 100,000 cm2/Vs
– Standard n-FET  = 1,500 cm2/Vs
• Carrier density (conductance) can by
electrostatically tuned
• Tunable field emission
CNT Chirality
• Graphite sheets have
2D E-k diagrams
• Semiconducting along
some vectors and
conducting along others
• CNT rolled from
graphite forces 1D E-k
behaviour
• Forces either
semiconducting or
conducting behaviour
a) Graphite Sheet
b) CNT made from graphite roll
Bulk Synthesis of CNTs
• CNTs are grown by bulk synthesis then
deposited on a substrate by spinning or drying
(liquid epitaxy)
– Arc Synthesis
– Laser-assisted Growth
Catalyst
Carbon sublimates onto catalyst
• Tubes are bundled together in “ropes” and are
highly tangled
• Must be cut apart before deposition
(ultrasonication)
• Creates tubes of varying lengths and many
defects
Growth of CNTs
• Nanotubes can be grown directly on the
substrate using CVD
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PECVD
Thermal CVD
Alcohol Catalytic CVD
Vapour Phase Growth (no substrate)
Aero gel-supported CVD
Laser-assisted thermal CVD
• SWNT diameter controllable
• Simple process on existing equipment
CNT Gas Sensors
• Carbon nanotubes can have extremely high Efields near the tip
• Great field emission
• Can be used to measure the discharge currents
of different gasses
Anode
Insulator
CNTs (Cathode)
Substrate
CNT Field-Emission Displays
• CNTs can shoot electrons at a
phosphorous screen
Phosphorous
CNTs
Insulator
Substrate
CNT Field-Effect Transistors
• CNT is used as the
channel between
source and drain
• Works as a FET
• Very small feature
size ideal for
advanced digital
circuits
CNT Force Measurement
• Use a CNT as a cantilever on an atomic
force microscope (AFM) to improve
resolution
AFM Cantilever
CNT
CNT Zoom Lenses
• CNT Index of refraction can be adjusted
with the application of an E-field (n ~0.9)
Convex Zoom Lens
Transparent Electrode
CNTs
Concave Zoom Lens
Variable Phase Shifter
Substrate
CNT Radiative Recombination
Thank You!
• This presentation will be available on the
web.