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

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:

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

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

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?

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

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

"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

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

+

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?

Cryo-Electronics:

Cq~

qkT

RSFQ

+ ++-

+VG

I ~ exp(-3qVG/kT)

for 3 charges on the MEM's tip

Nano-Mechanical Switch:

After M. A. Baldo

giant magneto-resistance

"spintronics"

These switches are made of metallic components and are of inherently low impedance

+

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

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

~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

Nano-transformers are feasible using tapered plasmonic transmission lines:

+ + +- - -

+ + +- - -

+ + +

++- - -

- -+++

- - -+ + +

- - -

- - -+ + +

High Impedance

Normal Impedance

solid electrolyte

Cu

Cu

-

+VG1nm

Electro-Chemically Driven Metallic Switch:

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?

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:

Gate

p+

n-

n+

The tunnelling FET, (TFET); a gate controlled Esaki Diode:

p+

n-

Gate

n+

+

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:

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)

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

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

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:

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.

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

+

"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

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

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.