The electron waveguide Y-branch switch A review and arguments for its use as a base for reversible logic

1
The electron waveguide Y-branch switchA review
and arguments for its use as a base for
reversible logic

• Erik Forsberg
• Joint Research Center of Photonics
• of the Royal Institute of Technology and Zhejiang
  University
• Hangzhou 310027, P. R. China
• ????????????
• erikf_at_zju.edu.cn

2
Outline

• Basic idea
• Theoretical
• Required switching voltage
• Single mode operation
• Ballistic switching
• Experimental
• Logic
• Reversible logic
• Conclusions

3
Electron Waveguide Y-Branch Switch (YBS)T. Palm
and L. Thylén, Appl. Phys. Lett. 60, 237 (1992)
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.
Required switching voltage

• no thermal limit ? promises extreme
  low-power consumption
• waveguide device ? small is good
• monotonic response ? tolerant to fabrication
  inaccuracies
• economics ?

4
Required switching voltageT. Palm, L. Thylen, O.
Nilsson, C. Svensson, J. Appl. Phys. 74, 687
(1993)
Required change in applied gate bias required to
change the state of the YBS

• Example (GaAs)
• Sheet carrier concentration 4x1015 m-2
• Interaction length 200 nm
• ? Theoretically required switch voltage 1 mV

Contrast
Sub-thermal switching in YBS just experimentally
verified ! L. Worschech et. al., private
communication
5
Electron transport Landauer-Büttiker formalism
Transmission probability stem ? right arm
6
Space-charge effects switching
The Self-Gating EffectJ-O J. Wesström Phys. Rev.
Lett. 82 2564 (1999)
7
Space charge contd...E. Forsberg, J. Appl.
Phys, 93, 5687 (2003)E. Forsberg and J.-O. J.
Wesström, Solid-State. Electron. 48, 1147-1154
(2004).
Fully self-consistent simulation tool for
simulations of electron waveguide devices
developed.

• 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.

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

8
Detecting selfgatingK. Hieke and M. Ulfward,
Phys. Rev. B 62, 16727 (2000).L. Worschech et.
al., Appl. Phys. Lett. 79, 3287 (2001).
Leave stem, W1, floating and measure its
potential while varying branch voltages
Theory then predicts
Expected result
Experimental result
9
Ballistic switching modeH. Q. Xu, Appl. Phys.
Lett. 78, 2064 (2001).
Three star coupled QPCs
10
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

11
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).

• Simple fabrication technique
• However
• Confinement too weak

12
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

13
Fabrication Schottky gatesE. Forsberg and K.
Hieke, Phys. Scri. T101, 158 (2002).

• Strong confinement ? single mode easily
  achieved
• Demonstrated in
• GaAs/AlGaAs
• Better gate efficiency possible
• However
• Complex fabrication technique

14
YBS-based circuits

• Fan-out possible
• Tolerant to fabrication defects
• Monotonic response
• Coherence only required in branching region

15
Logic Based on Y-branch Switches
T. Palm and L. Thylén, J. Appl. Phys. 79 8076
(1996) E. Forsberg, unpublished
Electrical symbol and possible states
Inverter
NAND gate using asymmetrical Y-branch switches
16
Ballistic YBS logic
S. Reitzenstein et. al., Electron. Lett. 38, 951
(2002)
H. Q. Xu, IEEE Electron. Dev. Lett 25, 164 (2004).
17
YBS logic

• Single mode operation logic
• Feasible
• Low power operation due to sub-thermal switching
• Advantage over CMOS FET ?
• Ballistic
• Demonstrated _at_ 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?

18
Comparing numbers
Switchenergy for a device with capacitive inputs
?Vswitch 1 mV C 0.1 pF
Minimum switch energy for typical YBS is thus of
the order 0.6 meV.
kBT ln 2 18 meV _at_ room temperature.
Conclusion Reversible logic can greatly reduce
the power dissipation of YBS-based logic.
19
Reversible YBS logicE. Forsberg, Nanotechnology
15, 298 (2004).
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
20
Implementation

• Possible today III-Vs
• 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).
21
TimingM. Frank et. al., private communication
22
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