Principles of Semiconductor Devices-lecture38

8/8/2019 Principles of Semiconductor Devices-lecture38

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Alam ECE-606 S08 1

EE-606:Lecture 38: Modern MOSFET

Muhammad Ashraful AlamElectrical and Computer Engineering

Purdue UniversityWest Lafayette, IN USA

Fall 2007

www.nanohub.org

NCN


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topic map

MOS-FET

BJT/HBT

Schottky

Diode

CircuitsLargeSignalSmallsignalDCEquilibrium


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outline

Short channel effect

Control of threshold voltage

Mobility enhancement

Conclusion

* 2( )

ch

xD t

ohG

CI V V

L

=

REF: Chapter 19, SDF


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short channel effect: Vth roll-off

L

Vth

Channel length

2 2 B A T th F F ox ox

Q qN W V

C C = = +


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short channel effect: punch-through

L

ln(ID)

VG

Longer channel

Short channel

Recall similar problem with bipolar transistor


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physics of short channel effect

,

,2

B Long

th L F

ox

QVC

=

L

L

,

,2

B Short

th Short F

ox

QV

C=

L

L

,

'

2

'

2

A T

B Short

A T

L LqN Z W

Q

Z LL L

qN WL

+

=

+

=

, B long A T Q qN W

, ,

,

1

'1

2

B Long B short

th

ox B Long

A T

ox

Q QV

C Q

qN W L L

C L

=

+ =


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short channel effect

2

2 2 '( )

2

2' 2 1 1

J S T J

TJ

J

L Lr W W r

W L L r

r

+ = + +

= +

L

L

'1

22

1 1

A T

th

ox

A T J T

ox J

qN W L LV

C LqN W r W

C L r

+ =

= +

rJ

Ws

WT


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how to make Vth roll-off small

min

2

1 1

A T J T

ox J

qN W r W

L C r

= +

Vth

LchLmin

!Shallow junction and geometry of transistorslaser annealing of junctions, FINFETs

! Reduced substrate doping NAConsider WT and junction breakdown

!Thinner gate oxides

Consider tunneling current! Higher gate dielectric

Consider bulk traps


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outline




Conclusion


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solution: ultra-thin body SOI

EF

VG

VG

tox

tSi

NA


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solutions: silicon on insulator


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Example: FINFET, OmegaFET, X-FET


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quantization and control of fin-width

EF

tSi

EC

EV

1

2

1h

2h

3h

n =

h2n22

2m*tSi2

EG = EG + 1 + 1h

x

E(e

V)

Band-gap wideningFluctuation in thickness


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21 1 A T J T

c

ox J

qN W r W L

C r

= +

Vth

LchLc

Shallow junction and geometry of transistorslaser annealing of junctions, FINFETs

Reduced substrate doping NAConsider WT and junction breakdown

Thinner gate oxides

Consider tunneling currentHigher gate dielectric

Consider bulk traps

(as small as possible)


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variability in Vth at low doping2 2B T

th F F

ox ox

DQ qN W

VC C

= =

From IBM Journal of Res. And Tech.


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variability in threshold voltage

2 2B Tth F F

ox ox

AQ qN W

VC C

= =

If every transistor has differentVth,and therefore different current,circuit design becomes difficult

* 2( )oxD G

ch

th

CI V V

L

=


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Vth control by substrate biasB

th s s s

ox

QV B

C = = +

2s F =

2s F BSV =


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Vth control by metal work-functionHigh-k/metal gate MOSFET

qs

qm

qVbi

EC

EV

EF

Vacuum level

( )bi g p M

FB

qV E

qV

= +

=

( )i ox G thQ C V V =

Bth FB s

ox

QV V

C= +


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0

0

21 1 A T J T c

ox J

qN W r W L

r

x

= +

Vth

LchLc

Shallow junction

laser annealing of junctionsHigher substrate doping NAConsider WT and junction breakdown

Thinner gate oxidesConsider tunneling current

Higher gate dielectricConsider bulk traps


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tunneling current

2

( ) ( )GqVi

T i G thA

n J Q V e T E

N

=

EC

EV

EFEG

T

ITRS


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0

0

21 1

ox

A T J T c

J

qN W r W L

r

x

= +

Shallow junction and geometry of transistors

laser annealing of junctions, FINFETSubstrate doping NAConsider WT and junction breakdown

Thinner gate oxidesConsider tunneling current

Higher gate dielectricConsider bulk traps

High-k/metal gate MOSFET


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advantages of high-k dielectric

0

0

21 1

ox

A T J T c

J

qN W r W L

rx

= +

High-k/metal gate MOSFET

* 2( )ox

c

D t

h

G hI V V

C

L=

Thicker oxide (x0) for same capacitance ensures the drive-current is not reduced, but tunneling current is suppressed.


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outline




Conclusion

* 2

( )ox

c

D t

h

G hI V V

C

L=


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Ge for PMOS, Si for NMOS

Takagi, TED 52, p.367, 2005


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basics of strain ..

Compressive biaxial strain

Enhances mobilityin the channel


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biaxial strain to enhance mobilty


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biaxial strain to enhance mobilty

Adapted from Chang et. al, IEDM 2005.


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Uniaxial strain to enhance mobilty


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summary

1) Short channel effect is a serious concern for

MOSFET scaling.

2) Many novel solutions at the material, device, circuit

level have been proposed to reduce short channel

effect.

3) The success of these efforts are now reflected in

effective MOSFET channel lengths of 30 nm.

Documents

Principles of Semiconductor Devices-lecture38