A Science & Technology Center

Theme 1: NanoElectronics

Second Annual E3S Retreat

August 28, 2011, Berkeley, CA

Eli Yablonovitch

Contra Costa-UC Berkeley-MIT-LATTC-Stanford-Tuskegee

A Science & Technology Center

The New Switch has to Satisfy Three Specifications:

1. Steepness (or sensitivity)

switches with only a few milli-volts

60mV/decade 1mV/decade

2. On/Off ratio. 106 : 1

10 : 10M

3. Current Density or Conductance Density

(for miniaturization)

old spec at 1Volt: 1 mAmp/micron

our spec: 1 milli-mho/micron

A 1 micron device should conduct at 1K in the on-state.

A Science & Technology Center

• Modulate the Tunneling Barrier

• Density of States Switch

2 Ways to Obtain Steepness

The sub-threshold slope for tunneling depends

on the steepness of the band-edges:

Conduction

band

Valence

band

Switching

Principle:

Conduction

band

Valence

band

Switching

Principle:

What could go wrong?

1. quantum-mechanical level repulsion:

En

erg

y L

evel

Gate Voltage

levels never line up!

What could go wrong?

2. The levels broaden due to the contacts:

conflicting requirements:

a. low contact resistance

b. sharp level

contact

b

Z

2

T4k

E2qT

hcontact conductance =

linewidth = (2/) EZ Tcontact

What could go wrong?

2. The levels broaden due to the contacts:

A compromise must be

accepted:

Tk

γ

8

2q

b

2

hGcontact conductance

Conductance

quantum

Penalty

for steep

response

1. Solve quantum-mechanical level repulsion problem:

En

erg

y L

evel

Gate Voltage

Ensure that contact broadening >

device,,

1T fZiZif EEM

device,,

1T fZiZif EEM

tunnel

matrix

element

1. Solve quantum-mechanical level repulsion problem:

Tcontact Tdevice

Requirement:

contact tunnel transmission is better than device tunnel transmission:

Tcontact >Tdevice

The device tunneling probablity

should neither be to big nor too small!

Tcontact

What else could go wrong?

3. The contact broadening is bad enough,

the also levels broaden due to the phonons

and they possibly develop side-bands

also called: band tails

also called: phonon assisted tunneling

It is embarrassing to the scientific world that we know

so little about this.

Both: theory is weak, and

experimental data are almost non-existent.

This science has to be a major goal of the Center

What else could go wrong?

3. The contact broadening is bad enough,

the also levels broaden due to the phonons

and they possibly develop side-bands

also called: band tails

also called: phonon assisted tunneling

It is embarrassing to the scientific world that we know

so little about this.

Both: theory is weak, and

experimental data are almost non-existent.

This science has to be a major goal of the Center

Type III band alignment

Idealized structure

90 95 100 105 110 115 120-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

En

erg

y, e

V

Distance, nm

Al0.60

Ga0.40

Sb

Ga0.60

In0.40

As0.364

Sb0.636

GaSb-QW

InAs-QW

AlSb

Real structure

Epitaxial design based only on available materials is possible

Zoomed view of the QW region with the AlSb barrier

Confinement level

for holes

Confinement level

for electrons

90 95 100 105 110 115 120-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

En

erg

y, e

V

Distance, nm

Al0.60

Ga0.40

Sb

Ga0.60

In0.40

As0.364

Sb0.636

GaSb-QW

InAs-QW

AlSb

Set of structures with different QW and barrier thicknesses

enables investigation of important aspects of tunneling physics

EC

OFF

material 2 (AlGaSb)

oxi

de

gate metal

qu

antu

m w

ell 1

(In

As)

EF

EV

EF

EC

ON

EF

EF

EV

oxi

de

gate metal

qu

antu

m w

ell 1

(In

As)

material 2 (AlGaSb)

EC

SHARPLY OFF

material 2 (AlGaSb)

oxi

de

gate metal

qu

antu

m w

ell 1

(In

As)

EF

EV

EF

CDEP

CQW

COX

z

A Density of States Switch is

explicitly affected by dimensionality:

I p

I

p

n

I p

n

I

p

I

p

n

I p

n

n

I

p

I

p

n

I p

1d:1d

1d:1d

0d:1d

0d:0d

2d:2d

2d:2d

n n

n

1d:2d

3d:3d 2d:3d

I

Z P

N

I P

N

Z LZ,i

I

VG

2OLVI

I

VG

N

I

P

Z

I

VG

N

I

P Z

X VG

OLVI 1

N

I

P

Z

I

VG

I

P

N

Z

I

VG

ConstantI

Y I

P

N

Z

Z I

P

X

N

I

VG

2/3OLVI

I

VG

OLVI

N

I

P

Z

I

VG

ConstantI

0 2 4 6 8 100

5

10

15

20

0 10 20 30 40 500

5

10

15

20

G

VOL(mV) (a)

Conduct

ance

(µS

)

G

VOL(mV) (b)

Conduct

ance

(µS

)

=2.34 meV

EZ=50 meV

Tdevice=2.16%

LX=32 nm

LZ=8.672 nm

m*=0.1

0 10 20 30 40 500

0.2

0.4

0.6

0 2 4 6 8 100

0.2

0.4

0.6

G/µm

VOL(mV) (a)

Conduct

ance

Den

sity

(m

S/µ

m)

G/µm

VOL(mV) (b)

Conduct

ance

Den

sity

(m

S/µ

m)

40

=2.34 meV

EZ=50 meV

Tdevice=2.16%

LX=32 nm

LZ=8.672 nm

m*=0.1

Case Picture Current Conductance, G Maximum G for pert.

theory to be valid

Maximum G

for end contacts

1d-1d N/A N/A

3d-3d N/A

N/A

2d-2dedge

N/A

N/A

0d-1d N/A

N/A

2d-3d N/A

N/A

1d-2d N/A

N/A

0d-0d

2d-2dface

1d-1dedge

T4k

qV

h

2q

3π

Vqm2L

b

deviceOL

2OL

*

X T

T4k

qE

2q

b

deviceZ T

h

T4k

qE

4q

2

qV

2π

Am

b

deviceZOL

2 T

h

T4k

qE

4qVqm

π

L

b

deviceZOL

*X T

h

T4k

qE

4q

bcontact

deviceiZ,

T

T

T4k

qEE

π

qmA

b

devicefZ,iZ,32 T

deviceOL

2OL

*

XV

h

2q

3π

Vqm2LT

deviceZE2q

Th

deviceZOL

2E

4q

2

qV

2π

AmT

h

deviceZOL

*X E4q

Vqmπ

LT

h

contact

deviceiZ,E

4q

T

T

devicefZ,iZ,32EE

π

qmAT

device

OL

fZ,iZ,22 qV

mEE

π

Lq2 T

T4k

q

qV

mEE

π

Lq2

b

device

OL

fZ,iZ,22 T

Tk

γπ

h

2q

b

22

22

b

232

π

2mA

T4k

1γ

2

π

h

2q

22

2

b

3/222

π

2mL

T4k

1γπ2

h

2q

Tk

γπ

h

2q

b

22

Tk

γ

4

2π

h

2q

b

3/22

Tk

γ

L

W

4

π

h

2q

bX

22

XE)(2/πγ

deviceOL

2

V2q

Th T4k

qV

2q

b

deviceOL

2

T

h

deviceOL

2

OL

2

*

V2q

2

qV

4π

AmT

h T4k

qV

2q

2

qV

4π

Am

b

deviceOL

2

OL

2

*

T

h

Owing to thickness fluctuations, we cannot rely upon

quantum well thickness uniformity to provide

reproducible device thresholds.

Solution: Employ monolayer quantum well materials

(like graphene, but with a bandgap like MoS2)

Search at the same time for more Type III

heterojunctions.

Eg VB

PbS 0.37 -5.11

FeAsS 0.2 -5.21

CuFeS2 0.35 -5.32

CuIn5S8 1.26 -5.35

Ag2S 0.92 -5.42

NiS 0.4 -5.43

CoAsS 0.5 -5.46

TiS2 0.7 -5.46

MnS2 0.5 -5.49

Cu2S 1.1 -5.54

Cu5FeS4 1 -5.55

CuInS2 1.5 -5.56

Pb5Sn3Sb2S14 0.65 -5.6

SnS 1.01 -5.67

NiS2 0.3 -5.69

Ce2S3 2.1 -5.69

In2S3 2 -5.7

Pr2S3 2.4 -5.83

HfS2 1.13 -5.84

PbCuSbS3 1.23 -5.84

FeS2 0.95 -5.87

MoS2 1.17 -5.9

Sb2S3 1.72 -4.72

MoS2 1.17 -4.73

PbS 0.37 -4.74

OsS2 2 -4.74

Cu3AsS4 1.28 -4.75

TiS2 0.7 -4.76

HgSb4S8 1.68 -4.81

WS2 1.35 -4.86

RuS2 1.38 -4.89

FeS2 0.95 -4.92

Pb5Sn3Sb2S14

0.65 -4.95

CoAsS 0.5 -4.96

CuFeS2 0.35 -4.97

MnS2 0.5 -4.99

FeAsS 0.2 -5.01

NiS 0.4 -5.03

NiS2 0.3 -5.39

PtS2 0.95 -5.53

Eg CB

Searching for

Type III heterojunctions

based on electronegativity:

top of the valence band

bottom of the

conduction band

with Hui Fang of Javey group

layered

compounds

Points for Discussion:

1. If we haven’t made the 2-terminal device

(Backward Diode) work,

shouldn’t we do that first before going on to the

3-terminal device (TFET).

2. We need a systematic study of level line shape in

the context of electron transport. E3S might be the

first ever to do this.

3. We need Type III band alignment.

Don’t we need a rational search, like the search

for high and low Work Function for metal gates?