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Page 1: Erik Forsberg Joint Research Center of Photonics

EF RC ’05 Ischia, Italy

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The electron waveguide Y-branch switch

A review and arguments for its use as a base for reversible logic

Erik ForsbergJoint Research Center of Photonics

of the Royal Institute of Technology and Zhejiang University

Hangzhou 310027, P. R. China 中国杭州浙江大学玉泉校区

[email protected]

Page 2: Erik Forsberg Joint Research Center of Photonics

EF RC ’05 Ischia, Italy

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Outline

• Basic idea• Theoretical

– Required switching voltage– Single mode operation– Ballistic switching

• Experimental• Logic• Reversible logic• Conclusions

Page 3: Erik Forsberg Joint Research Center of Photonics

EF RC ’05 Ischia, Italy

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Electron Waveguide Y-Branch Switch (YBS)T. Palm and L. Thylén, Appl. Phys. Lett. 60, 237 (1992)

e-

1

2 3 Single mode coherent mode of operation:

Envelope of electron wavefunction propagates to either drain depending on the direction of electric field across the branching region.

no thermal limit promises extreme low-power consumption

waveguide device small is good

monotonic response tolerant to fabrication inaccuracies

economics … ?

Tswitch eV

Required switching voltage:

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EF RC ’05 Ischia, Italy

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Example (GaAs): Sheet carrier concentration 4x1015 m-2

Interaction length 200 nm

Theoretically required switch voltage 1 mV

Required switching voltageT. Palm, L. Thylen, O. Nilsson, C. Svensson, J. Appl. Phys. 74, 687 (1993)

T

YBSS eV

Required change in applied gate bias required to change the state of the YBS:

Sub-thermal switching in YBS just experimentally verified !L. Worschech et. al., private communication

Contrast: e

TkV BFETS )10log(

Page 5: Erik Forsberg Joint Research Center of Photonics

EF RC ’05 Ischia, Italy

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Electron transport – Landauer-Büttiker formalism

-10 0 100

1

Gate bias [arb. units]

4

1

4

1

2

14

1

4

1

2

12

1

2

10

22

22

YT

eTER

I rY

r )(1

0

S

gg

V

V tanh

Transmission probability stem right arm

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EF RC ’05 Ischia, Italy

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Space-charge effects switching

e-

1

2 3

4

1

4

1

2

14

1

4

1

2

12

1

2

10

22

22

YT

rY

r VTER

I )(1

0

The Self-Gating EffectJ-O J. Wesström Phys. Rev. Lett. 82 2564 (1999)

S

gg

V

V tanh

S

sggg

V

WV 23tanh

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EF RC ’05 Ischia, Italy

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Space charge cont’d...E. Forsberg, J. Appl. Phys, 93, 5687 (2003)E. Forsberg and J.-O. J. Wesström, Solid-State. Electron. 48, 1147-1154 (2004).

Space-charge can be dominant. Dependence is complex. Single parameter model not adequate to model space charge effects Screening of gate voltage can be severe.

Fully self-consistent simulation tool for simulations of electron waveguide devices developed.

0

50

100

150

200 0

50

100

150

2000

1

x 10-3

[nm][nm]

[C/m

2 ]

0

50

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150

200 0

50

100

150

2000

1

x 10-3

[nm][nm]

[C/m

2 ]

Conclusions: Small charge densities allows for original response Gate efficiency is a showstopper

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EF RC ’05 Ischia, Italy

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Detecting selfgatingK. Hieke and M. Ulfward, Phys. Rev. B 62, 16727 (2000).L. Worschech et. al., Appl. Phys. Lett. 79, 3287 (2001).

-6 -4 -2 -0 2 4 6

Branch voltage [a.u.]

0

1

2

3

4

5

Ste

m v

olt

age

[a.u

.]Leave stem, W1, floating and measure it’s potential while varying branch voltages

Setrr WW 23

rr WW 21 Theory then predicts:

2223 1 rWW

Expected result

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

-0.4 -0.2 0 0.2 0.4

right drain voltage (V)

stem

vo

ltag

e (V

)

200 nm

320 nm

Experimental result

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EF RC ’05 Ischia, Italy

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Ballistic switching modeH. Q. Xu, Appl. Phys. Lett. 78, 2064 (2001).

Three star coupled QPCs

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EF RC ’05 Ischia, Italy

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Recap

YBS has three modes of operation • Single mode transport

– No thermal limit to switch voltage

• Self-gating operation– Switching based on space charge effects– Bi-stable mode of operation– (single mode operation)

• Ballistic switching – Multimode mode of operation– Room temperature operation demonstrated

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EF RC ’05 Ischia, Italy

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Fabrication: Split-gateP. Ramvall, P. Omling, T. Palm, and L. Thylen, "Quantum Confinement: Physics and Application" (Eds. M. Cahay et. al.) (The Electrochemical Society, Inc., 1994).

E lectron gas

Split gate

Simple fabrication technique

However… Confinement too weak

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EF RC ’05 Ischia, Italy

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Fabrication: In-plane gatesJ. O. Wesström et. al., "Quantum IV: Nanoscale Materials, Devices and Sytems" (Eds. M. Cahay et. al.) (The Electrochemical Society, Inc., 1997).L. Worschech et. al., Appl. Phys. Lett. 78, 3325 (2001).L. Worschech et. al., Physica E 12, 688 (2002).G. M. Jones et. al., Appl. Phys. Lett. 86, 073117 (2005).

Simple fabrication technique Strong confinement single mode easily achieved Demonstrated in

– GaAs/AlGaAs– InGaAs/InP– InAs/AlSb

However… Low gate efficiency

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EF RC ’05 Ischia, Italy

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Fabrication: Schottky gatesE. Forsberg and K. Hieke, Phys. Scri. T101, 158 (2002).

Pt:Schottky-gate

E lectron gas

Strong confinement single mode easily achieved Demonstrated in

– GaAs/AlGaAs Better gate efficiency possible

However… Complex fabrication technique

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EF RC ’05 Ischia, Italy

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YBS-based circuits

Fan-out possible Tolerant to fabrication defects

- Monotonic response- Coherence only required in branching region

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EF RC ’05 Ischia, Italy

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Logic Based on Y-branch Switches

Inverter

NAND gate using asymmetrical Y-branch switches

S

D1

G

D2

1011

0101

0010

0000

D2D1GS

Electrical symbol and possible states

T. Palm and L. Thylén, J. Appl. Phys. 79 8076 (1996)E. Forsberg, unpublished

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EF RC ’05 Ischia, Italy

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Ballistic YBS logic

S. Reitzenstein et. al., Electron. Lett. 38, 951 (2002) H. Q. Xu, IEEE Electron. Dev. Lett 25, 164 (2004).

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EF RC ’05 Ischia, Italy

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YBS logic

Single mode operation logic– Feasible – Low power operation due to sub-thermal switching– Advantage over CMOS FET ?

Ballistic – Demonstrated @ room temperature– Thermally limited – Advantage of CMOS FET ? – Feasible application: easy integration with III-V

semiconductor lasers/modulators Conclusion:

For conventional logic it is highly questionable if the YBS can ever outperform CMOS FETs in an economically competitive manner.

Other ideas?

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EF RC ’05 Ischia, Italy

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Comparing numbers

Switchenergy for a device with capacitive inputs: 2switchswitch VCE

ΔVswitch = 1 mV

C = 0.1 pF

Conclusion: Reversible logic can greatly reduce the power dissipation of YBS-based logic.

Minimum switch energy for typical YBS is thus of the order 0.6 meV.

kBT ln 2 = 18 meV @ room temperature.

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EF RC ’05 Ischia, Italy

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Reversible YBS logicE. Forsberg, Nanotechnology 15, 298 (2004).

A A '

B B '

C C '

A B C A’ B’ C’

0 0 0 0 0 0

0 0 1 0 0 1

0 1 0 0 1 0

0 1 1 0 1 1

1 0 0 1 0 0

1 0 1 1 1 0

1 1 0 1 0 1

1 1 1 1 1 1

ccNOT (Fredkin) gate

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EF RC ’05 Ischia, Italy

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Implementation

Possible today III-V’s However, present fabrication techniques limited

– Cryogenic operation required– Low gating efficiency Power dissipation due to information erasure not dominant

Other possibilities– Hexogonal networks – feasible– Carbon nanotubes – possible – Si nanowires – ?

A. N. Andriotis et. al., Appl. Phys. Lett. 79, 266 (2001).

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TimingM. Frank et. al., private communication

Moving periodic globalpotential

x

y

x

V

BasicY-junction“switch gate”Control

waveguide

Data waveguide

Electrostatic repulsion

Ground-state high- probability regions of two electrons’ wave packets

Left branch

Rightbranch

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EF RC ’05 Ischia, Italy

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Summary

• YBS summarized– Recap on theoretical work – Summary of experimental work– Conventional logic based on YBS

• Reversible logic based on YBS

• The road ahead– Clocking schemes etc– Feasible designs– Fabrication issues– Gating efficiency potential showstopper invariant of

implementation technology