Mesoscale Bulk ElectronicsMesoscale Bulk Electronics

Beyond the MOSFETBeyond the MOSFET• MesoscaleMesoscale::

– An intermediate scale, on the order of ~10 nm, An intermediate scale, on the order of ~10 nm, – Materials have some properties of bulk material,Materials have some properties of bulk material,– But surface effects are important,But surface effects are important,– And more quantum phenomena become importantAnd more quantum phenomena become important

• BulkBulk::– Materials & structures fabricated using bulk processes, w/o Materials & structures fabricated using bulk processes, w/o

atomic precisionatomic precision

• ElectronicsElectronics::– Electron states are used for primary information-processing Electron states are used for primary information-processing

operationsoperations• not photons (optical), or whole atoms (mechanical)not photons (optical), or whole atoms (mechanical)

What happens @ mesoscale?What happens @ mesoscale?• MOSFET scaling hampered by quantization of:MOSFET scaling hampered by quantization of:

– charge: charge: • becomes important @ becomes important @ LL 10 nm in all materials 10 nm in all materials

– energy levels: energy levels: • important in semiconductors @ important in semiconductors @ L L 10 nm 10 nm

• Can alternative device operating principles Can alternative device operating principles exploitexploit these quantization effects rather than be hampered by these quantization effects rather than be hampered by them?them?

• Some approaches:Some approaches:– Single-electron transistorsSingle-electron transistors– Quantum wells / wires / dots, quantum-dot CAsQuantum wells / wires / dots, quantum-dot CAs– Resonant tunneling diodes / transistorsResonant tunneling diodes / transistors

““Coulomb blockade effect” SETCoulomb blockade effect” SET• Based on charge quantizationBased on charge quantization• # of energy levels may not be noticeably quantized# of energy levels may not be noticeably quantized• Comprised of an island of (typically) metal Comprised of an island of (typically) metal

surrounded by insulator.surrounded by insulator.• Narrow tunnel junctions to transistor source/drain - Narrow tunnel junctions to transistor source/drain -

5-10 nm typical 5-10 nm typical• Gate controls # of electrons that may occupy islandGate controls # of electrons that may occupy island

– within precision of 1, out of millionswithin precision of 1, out of millions

Quantum Wells/Wires/DotsQuantum Wells/Wires/Dots• Usually use semiconductor material.Usually use semiconductor material.• Electron position is narrowly confined in 1, 2, or 3 Electron position is narrowly confined in 1, 2, or 3

dimensions, respectively.dimensions, respectively.EE between distinct momentum states becomes between distinct momentum states becomes

largelarge• In quantum dots, total # of mobile electrons may be In quantum dots, total # of mobile electrons may be

as small as 1!as small as 1!• Non-transistorlike quantum-dot logics:Non-transistorlike quantum-dot logics:

– Most notably, quantum dot “cellular automata” by Most notably, quantum dot “cellular automata” by Notre Dame groupNotre Dame group

Quantized Energy LevelsQuantized Energy Levels• The narrower the space, The narrower the space,

– the smaller the the smaller the gap between normal gap between normalmodes #modes #nn and # and #nn+1+1

– the the largerlarger the frequency & energy gap the frequency & energy gapbetween those modesbetween those modes

• More confinedMore confinedspaces have widerspaces have widerenergy gaps betweenenergy gaps betweentheir distincttheir distinctmomentum states.momentum states.

, E

/2, 2E

/3, 3E

Resonant Tunneling DiodesResonant Tunneling Diodes• Usually based on quantum wells or wiresUsually based on quantum wells or wires

– 1-2 effectively “classical” degrees of freedom1-2 effectively “classical” degrees of freedom

Source Drain

Island (narrow bandgap)

Tunnel barriers (wide bandgap)

Occupied states inconduction band

Energy

Quantized momentum state

Electron tunnelsthrough barrier

Electron flow Unoccupied states

Resonant Tunneling TransistorsResonant Tunneling Transistors• Like RTDs, but an adjacent gate electrode helps Like RTDs, but an adjacent gate electrode helps

adjust the energy levels in the islandadjust the energy levels in the island

Source Drain

Gate

Future Semiconductor StructuresFuture Semiconductor Structures• SOI (Silicon-on-Insulator)SOI (Silicon-on-Insulator)• Band-engineered transistorsBand-engineered transistors• Vertical transistorsVertical transistors• FinFETs (Chenming Hu group @ Berkeley)FinFETs (Chenming Hu group @ Berkeley)• Double-gate transistors (Double-gate transistors (e.g.e.g. Philip Wong IBM) Philip Wong IBM)• Multi-layer chips (Lee @ Stanford)Multi-layer chips (Lee @ Stanford)• ““Quantum FET” analysis (Merkle ‘93)Quantum FET” analysis (Merkle ‘93)• atom-width wires (need ref)atom-width wires (need ref)

Go through ITRS presentation

Nanoelectronics TechnologiesNanoelectronics Technologies• Scaled MOSFET structures - prev. slideScaled MOSFET structures - prev. slide• Quantum wells/wires/dots - covered last timeQuantum wells/wires/dots - covered last time

– quantum dot cellular automata - go thru quantum dot cellular automata - go thru websitewebsite

• Various “single-electron” devices - todayVarious “single-electron” devices - today• ““Spintronics” - electron (&/or nuclear?) spin Spintronics” - electron (&/or nuclear?) spin

based electronics- todaybased electronics- today• Molecular electronics - today or FridayMolecular electronics - today or Friday

Quantum Dot Cellular AutomataQuantum Dot Cellular Automata• Wires: x vs +, fan-out, wire-crossingWires: x vs +, fan-out, wire-crossing• Speed: 2 ps/cellSpeed: 2 ps/cell

– compare: light can go 0.6 mm in 2 pscompare: light can go 0.6 mm in 2 ps– ordinary electronic signals ~0.3 mmordinary electronic signals ~0.3 mm– MOSFET gate delay according to ITRS ‘99: MOSFET gate delay according to ITRS ‘99:

• 11 ps in ‘99, 5.7 in ‘05, 2.4 in ‘1411 ps in ‘99, 5.7 in ‘05, 2.4 in ‘14

• Gates: inverters, majority gates, full adderGates: inverters, majority gates, full adder• Paradigms:Paradigms:

– ground state computingground state computing– clocked QCA pipelining (adiabatic, reversible)clocked QCA pipelining (adiabatic, reversible)

• Molecular version: 20 fs/cell (100x smaller)Molecular version: 20 fs/cell (100x smaller)

SpintronicsSpintronics• Cf. Das Sarma group at UFCf. Das Sarma group at UF• Info written into spin orientation of electronsInfo written into spin orientation of electrons

– persists for nanoseconds in conduction e’spersists for nanoseconds in conduction e’s– compare ~10 fs lifetime for momentum decaycompare ~10 fs lifetime for momentum decay

• Spin control, propagation along wires, Spin control, propagation along wires, selection, & detectionselection, & detection

• Datta-Das and Johnson spin-based transistorsDatta-Das and Johnson spin-based transistors• Potential medium for quantum computationPotential medium for quantum computation

UF contacts: Arthur Hebard,

Jeff Krause

Molecular ElectronicsMolecular Electronics• Tour wiresTour wires• Molecular switchesMolecular switches• Carbon nanotube devicesCarbon nanotube devices

Helical LogicHelical Logic• Proposal by Merkle & Drexler ‘96Proposal by Merkle & Drexler ‘96• Do w. conductors & insulators only!Do w. conductors & insulators only!

– no fancy semiconductors, superconductors, or tunnel no fancy semiconductors, superconductors, or tunnel junctions needed...junctions needed...

• The wires The wires areare the devices! the devices!– Uses simple Coulombic repulsion Uses simple Coulombic repulsion

between electrons to do logic between electrons to do logic

• Scalable to single electrons & atom-wide wires!Scalable to single electrons & atom-wide wires!• Externally clocked...Externally clocked...

– by rotation of CPU within a fixed electrostatic fieldby rotation of CPU within a fixed electrostatic field

• Can be used reversibly… 10Can be used reversibly… 10-27-27 J, 1K, 10 GHz! J, 1K, 10 GHz!

See plastictransparencies,

readingsfor details

HL: Overall Physical StructureHL: Overall Physical Structure• Consider a cylinder of sparse (high-permissivity) insulating material

(e.g., air), containing embedded helical coils of cold conductive or semiconductive wire, rotating on its axis in a static, flat electric field (or, unmoving in a rotating field).

• An excess of conduction electronswill be attracted to regions on wire closest to + fielddirection.

• These electron packetsfollow the field along as itrotates relative to thecylinder.

• Next slide: Logic!

Switch gate operation: 1 of 3Switch gate operation: 1 of 3

Datawire

Conditionwire

Switch gate operation: 2 of 3Switch gate operation: 2 of 3

Coulombicrepulsion

Datawire

Conditionwire

Switch gate operation: 3 of 3Switch gate operation: 3 of 3

Datawire

Conditionwire

Nano-mechanical logicsNano-mechanical logics• First proposed by Drexler, 1992 (& earlier)First proposed by Drexler, 1992 (& earlier)

– Typically, very low leakage!Typically, very low leakage!• due to high energy barriers (mechanical rigidity) in interactions due to high energy barriers (mechanical rigidity) in interactions

involving bonded atoms, vs. just electronsinvolving bonded atoms, vs. just electrons– PrettyPretty fast due to small size, but probably... fast due to small size, but probably...

• ~1000’s × slower than molecular electronics might be~1000’s × slower than molecular electronics might be– basically, because atoms are ~1000’s × heavier than electronsbasically, because atoms are ~1000’s × heavier than electrons

• Drexler’s logic of rods, cams, springsDrexler’s logic of rods, cams, springs– Molecular scale componentsMolecular scale components

• Covalently bonded, atomically preciseCovalently bonded, atomically precise

• Merkle’s (1993) “buckling” logicMerkle’s (1993) “buckling” logic– No sliding-contact interfacesNo sliding-contact interfaces– Scalable from macroscale to mesoscaleScalable from macroscale to mesoscale

Also seeSmith’splanar

mechanicallogics

See plastictransparencies,

readingsfor details

Molecular ElectronicsMolecular Electronics• Tour wiresTour wires• Various molecular switchesVarious molecular switches• Various carbon nanotube devicesVarious carbon nanotube devices

• Potential problem areas:Potential problem areas:– High resistance of existing molecular devices.High resistance of existing molecular devices.– Maintaining thermal reliability in face of low node Maintaining thermal reliability in face of low node

capacitances and voltages.capacitances and voltages.– High leakage currents, due to tunneling or thermal High leakage currents, due to tunneling or thermal

excitation over small, narrow barriers.excitation over small, narrow barriers.

See plastictransparencies,

readingsfor details

Biochemical computingBiochemical computing• Selected points on DNA computing:Selected points on DNA computing:

– Adleman’s experimentAdleman’s experiment– Cyclic Mixture MutagenesisCyclic Mixture Mutagenesis

• Reversible DNA Turing MachinesReversible DNA Turing Machines– Seeman’s self-assembling structuresSeeman’s self-assembling structures– Winfree’s tile self-assembly logicsWinfree’s tile self-assembly logics

• DNA computing has many disadvantages:DNA computing has many disadvantages:– High cost of materialsHigh cost of materials– Slowness of diffusive molecular interactionsSlowness of diffusive molecular interactions– Slowness/cost/unreliability of lab stepsSlowness/cost/unreliability of lab steps

• Prob. won’t Prob. won’t everever be a cost-effective computing be a cost-effective computing paradigm (except paradigm (except maybemaybe for for in vivoin vivo apps) apps)

Seereadings

for details

Optical computingOptical computing• Not viable at the nanoscale anytime soon!Not viable at the nanoscale anytime soon!

– Due to entropy density issues mentioned earlier:Due to entropy density issues mentioned earlier:• High enough info. flux requires extremely energetic photons, High enough info. flux requires extremely energetic photons,

with too-high effective temperatureswith too-high effective temperatures• Or, waveguides considerably smaller than photon wavelengths - Or, waveguides considerably smaller than photon wavelengths -

EMF theory suggests: Impossible!EMF theory suggests: Impossible!

• All-optical computing requires All-optical computing requires nonlinearnonlinear interactions, interactions, between photons & materials.between photons & materials.

• Optics (or more generally, EMF waves) will remain Optics (or more generally, EMF waves) will remain useful for communications, but useful for communications, but onlyonly::– in contexts where extreme bandwidth density is not in contexts where extreme bandwidth density is not

required (or extreme temperatures can be tolerated)required (or extreme temperatures can be tolerated)