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Advanced Topics In Solid State Devices EE290B
Will a New Milli-Volt Switch Replace the Transistor for Digital Applications?
August 28, 2007
Prof. Eli YablonovitchElectrical Engineering & Computer Sciences Dept.
University of California, Berkeley, CA 947201
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processedbitperkT40~ConsumedEnergy
fkT4qkT2fq2iVpowerRequired
fq2i
fq2i
fiq2iRatioNoisetoSignal
mVolts50~q/kT2iRVvoltageRequired
NoiseJohnson..........fRkT4i
NoiseShot...........fiq2i
2
2
Read the current going through a resistor, in the presence of noise:
With a safety margin:
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What will be the energy cost, per bit processed?
1. Logic energy cost ~40kT per bit processed
2. Storage energy cost ~40kT per bit processed
3. Communications currently >100,000kT per bit processed
.
3
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There are many type of memory possible:1. Flash2. SRAM3. Dram4. Magnetic Spin5. Nano-Electro-Chemical Cells6. Nano-Electro-Mechanical NEMS7. Moletronic8. Chalcogenide glass (phase change)9. Carbon Nanotubes
Similarly there are many ways to do logic.
But there are not many ways to communicate:
1. Microwaves (electrical)2. Optical 4
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fkTR
V
fRkTV
noise
noise
4
42
2
What is the energy cost for electrical communication?
Signal Noise PowerEnergy per bit = 4kT per bit
All information processing costs ~ 40kT per bit. (for good Signal-to-Noise Ratio)
Great!
So what’s the problem?
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mVolt1V
CqqkT4V
Cq
qkT4V
C1kT4V
RC1RkT4V
fRkT4V
Volts10mVolts100noise
2noise
2noise
2noise
2noise
//
The natural voltage range for wired communication is rather low:
The thermally activated device wants at least one electron at ~1Volt.
The wire wants1000 electrons at 1mVolt each.
(to fulfill the signal-to-noise requirement >1eV of energy)
Voltage Matching Crisis at the nano-scale!
If you ignore it the penalty will be (1Volt/1mVolt)2 = 106
The natural voltage range for a thermally activated switch like transistors is >>kT/q, eg. ~ 40kT/q
or about ~1Volt
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10μm
1μm
100nm
10nm
Moore's Law
1960 1980 2000
Crit
ical
D
imen
sion
2020 2040 2060Year
Tech
nolo
gy G
ap
Gates only
Gates including wires
107
105
102
1
0.1
104
106
10
108
103
Ene
rgy
per B
it fu
nctio
n (k
T)
The other , for energy per bit function
Shoorideh and Yablonovitch, UCLA 2006
Transistor Measurements by Robert Chau, Intel
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"In addition, power is needed primarily todrive the various lines and capacitances associated with thesystem. As long as a function is confined to a small area ona wafer, the amount of capacitance which must be driven isdistinctly limited. In fact, shrinking dimensions on an integratedstructure makes it possible to operate the structure athigher speed for the same power per unit area."
p. 114
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nano-transformerh
~1eV
A low-voltage technology, or an impedance matching device,needs to be invented/discovered at the Nano-scale:
transistor amplifier with steeper sub-threshold slope photo-diode
+ ++-
+VG
MEM's switch
Cryo-ElectronicskT/q~q/C
Cu
Cu
solid electrolyte
Electro-Chemical Switch
giant magneto-resistancespintronics
+
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An amplifying transistor as a voltage matching device:Small voltage inLarge voltage out
in
outAmplification of weak signals has an energy cost!Amplification of weak signals has a speed penalty!
ln{I}
VgGate Voltage
Cur
rent
steep
ersu
b-th
resh
old
slope
correlated electron motion?
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Cryo-Electronics:
Cq~
qkT
RSFQ
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+ ++-
+VG
I ~ exp(-3qVG/kT)
for 3 charges on the MEM's tip
Nano-Mechanical Switch:
After M. A. Baldo
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giant magneto-resistance
"spintronics"
These switches are made of metallic components and are of inherently low impedance
+
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Si (001) SubstrateTa 5nm
Ru 50nm
Ta 5nm
NiFe 5nm
Antiferromagnetic MnIr 8nm
CoFe 2nm
Ru 0.8nm
Ferromagnetic CoFeB 3nm
MgO 1.5nm Tunnel Barrier
Ferromagnetic CoFeB 3nm
Ta 5nm
Transpinnor Structure:
Ru 15nm
6:1 Resistance Change inTunnel Magnetoresistive (TMR)
stack[1]
Current Gate
Isignal
Magnetization
Drain
Source
InsulatorCurrent Gate
Drain
Source
Isignal
[1] Ikeda et. al., Japanese Journal of Applied Physics, Vol. 44, No 48, pp. L1442-L1445
BField
BField
Device Area 1μm2
Gate
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5μA output5μA input
Complementary Transpinnor Logic
500Ωor
2.275kΩ
2.275kΩor
500Ω
+V +3mV
-V -3mV
Output Power = 1.6*10-8 WTotal Power = 2.5*10-8 W
Efficiency=65%
•Problems: On/Off ratio is only about 5:1 Still takes too many Amps to switch
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~1eV
A Photo-Diode for impedance matching:
h
h
Problems:
• Ten photons are needed to assure a good Bit Error Ratio.
• Source efficiency may be only 10%.
Energy penalty 100 × 40kT
nhnh
• multi-photon millimeter-wave "optical"
• plasmonic wires
Miller DAB "Optics for Low-Energy Communication Inside Digital processors – Quantum Detectors, Sources, and Modulators as Efficient Impedance Converters Optics Letters 14 (2): 146-148 JAN 15 1989
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Nano-transformers are feasible using tapered plasmonic transmission lines:
+ + +- - -
+ + +- - -
+ + +
++- - -
- -+++
- - -+ + +
- - -
- - -+ + +
High Impedance
Normal Impedance
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solid electrolyte
Cu
Cu
-
+VG1nm
Electro-Chemically Driven Metallic Switch:
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An amplifying transistor as a voltage matching device:Small voltage inLarge voltage out
in
outAmplification of weak signals has an energy cost!Amplification of weak signals has a speed penalty!
ln{I}
VgGate Voltage
Cur
rent
steep
ersu
b-th
resh
old
slope
correlated electron motion?
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EF
The sub-threshold slope for tunneling depends
on the steepness of the band-edges:
band
to b
and
tunn
elin
gBias Voltage
Diffu
sion
curr
ent
Sharp Step
Curr
ent
The Esaki Diode as a Switch:
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Gate
p+
n-
n+
The tunnelling FET, (TFET); a gate controlled Esaki Diode:
p+
n-
Gate
n+
+
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EF
The sub-threshold slope for tunneling depends
on the steepness of the band-edges:
band
to b
and
tunn
elin
gBias Voltage
Diffu
sion
curr
ent
Sharp Step
Curr
ent
The Gate Controlled Esaki Diode:
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The optical absorption coefficient,(h), of Si at 300K, in the vicinity of the band edge.
The Urbach edge grows as:(h) ~ exp{(h-Eg)/Eo},
where the Eo parameter is a type of sub-threshold slope.
Eo ~ 10meV for Silicon
It's good, but it should be better. We need to search for materials with steeper band-edges!
Tom Tiedje, Eli Yablonovitch, George D. Cody, and Bonnie G. Brooks IEEE Trans. On Elec. Dev., VOL. ED-31, NO. 5 , p. 711 (MAY 1984)
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E
Ec
EvEnergy level fluctuations = (% strain) Deformation
Potential
Energy level fluctuations = 0.5% 5eV 25meV
Why aren't the band edges sharp? – thermal strains & Deformation Potentials
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0.0 0.2 0.4 0.6 0.8 1.0500
600
700
800
900
GeSi
X
L
Con
duct
ion
Ban
d En
ergy
(meV
)
Si1-x
Gex
Short-Period L&XSuperlattice
eV 54.131 :Ge
eV 18.431 :Si
Lu
Ld
ud
Surprisingly, Si & Ge have opposite sign Deformation Potentials:
Cancelling the effects of the thermal sound waves might be possible:Deformation Potential Engineering?
Deformation Potentials:
opposite
sign
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Gate
Si Ge
p+
n-
n+
The challenge is to cancel the effects of: isotropic strainuni-axial strainshear strain
Construct the channel as a short-period Si-Ge superlattice:
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If Vdd is lower, then the current drive requirements go down too!
Some of the tunneling transistors have a relatively low current drive capability:
Current Density Requirement:
For conventional transistors: 1 milli-Amp / micron
If a lower operating voltage is achieved, then the current required to charge the wires would also be lower.
A Figure-of-Merit that allows for this possibility would be expressed as: 1 milli-Siemen / micron
A current density as low as 1 micro-Amp / micron would be acceptable, if the operating voltage were as low as 1 milli-Volt.
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nano-transformerh
~1eV
A low-voltage technology, or an impedance matching device,needs to be invented/discovered at the Nano-scale:
transistor amplifier with steeper sub-threshold slope photo-diode
+ ++-
+VG
MEM's switch
Cryo-ElectronicskT/q~q/C
Cu
Cu
solid electrolyte
Electro-Chemical Switch
giant magneto-resistancespintronics
+
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"In addition, power is needed primarily todrive the various lines and capacitances associated with thesystem. As long as a function is confined to a small area ona wafer, the amount of capacitance which must be driven isdistinctly limited. In fact, shrinking dimensions on an integratedstructure makes it possible to operate the structure athigher speed for the same power per unit area."
p. 114
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10μm
1μm
100nm
10nm
Moore's Law
1960 1980 2000
Crit
ical
D
imen
sion
2020 2040 2060Year
Tech
nolo
gy G
ap
Gates only
Gates including wires
107
105
102
1
0.1
104
106
10
108
103
Ene
rgy
per B
it fu
nctio
n (k
T)
The other , for energy per bit function
Shoorideh and Yablonovitch, UCLA 2006
Transistor Measurements by Robert Chau, Intel
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Recommendations:
1. Milli-Volt powering should be regarded as a Goal for future electronic switching devices.
2. There would be both an immediate benefit, as well as a benefit at the end of the roadmap.
3. Band edge steepness is poorly known, and should be investigated for a number of semiconductors and semi-metals.
4. The full range of technology options should be included.