device scaling

7/24/2019 Device Scaling

1/19

Device Scaling: CMOS

Dipankar Pal,

Dept. of EEE & EI,

BITS-Pilani, K. K. Birla Goa Camp!


2/19

Scaling of "#S concern! !$!tematic re%ction of

%imen!ion! !pporte% $ availale tec'nolog$.

Features:

Geometric ratio! are pre!erve%.

Proportional !caling re!lt! in a re%ction of !ilicon area.

Increa!e! overall fnctional %en!it$ of c'ip.

Total po(er %i!!ipation i! re%ce% in Constant Field !caling)

increa!e% inConstant Voltage !caling

MOSFET IN SMALL-GEOMETRY

Ref: "CMOS Digital Integrated Circuits:

Analysis & Design", Kang, S. et al


3/19

Trend in Scaling of technology



*ear + +/ + ++ +0 + +/ +

Feature

Size !#

$.% .' .$ .( (.) (.% (.*% (.$%

A+erage d-nscaling fr a generatin t ne/t is 0y a factr S: 1 .$ t .%#2 3a44ense+ery $5* years . 6day (.() ! r )n# tec3nlgy is cn and -idely used.

Scaling of a typical MOSFET by a factor of S


4/19

Method of Scaling



Constant Field Scaling (Full Scaling)

To scale by a constant factor S (> 1):

All hori!ontal and "ertical di#enion of

di"ided by S$

S%pplie caled by a#e factor &S'

The e(tent of caling &S' deter#ined by

technology and #ini#%# feat%re i!e$


5/19

Components after Scaling (Quantitative)



( ) ( )

22

'''

2

2

2'''

'

..1

.

,

.

)(.

1.

2

..

2)(

S

PVI

SVIP

Now

VIP

S

satIVV

S

kSVV

ksatI

DSDDSD

DSD

DTGS

nTGS

nD

===

=

===( )[ ]

( )[ ]S

linIVVVV

S

kS

VVVVk

linI

CSt

St

C

DDSDSTGS

n

DSDSTGSn

D

ox

ox

ox

ox

oxox

)(..2.

1.

2

.

..2.2

)(

..

2

2

2'''''

'

'

'

==

=

===


6/19

Method of Scaling



Constant Voltage Scaling

To scale by a constant factor S (> 1):

All horizontal and vertical dimensions divided by S.

Extent of scaling (S) determined by technology and minimum feature size.

Sulies !et unchanged.

"oing densities increased by a factor S#


7/19



Components after Scaling (Quantitative)

( )[ ]

( )[ ] )(...2.2

.

..2.2

)(

2

2''''

'

'

linISVVVVkS

VVVVk

linI

DDSDSTGS

n

DSDSTGSn

D

==

=

( ) ( )

PSVISVIP

satISVVkS

VVk

satI

DSDDSD

DTGSn

TGSn

D

....

)(..2

..

2)(

'''

22'''

'

===

===


8/19

Shortco#ing of Scaling - I

$radual %hannel Aroximation ($%A) no more holds.

&f channel length (') on the order of "rain Source deletion

region (xd" xdS) or deth of "rain Source*+unction(x+) thedevice is called short channel device.




9/19

Short )hannel Effect



Li#itation on electron drift characteritic

Red%ction in threhold "oltage

Electron *rift )haracteritic

+$ Increaed channel electric field &Ey , +. /0c#' at%rate drift

"elocity &"d&at' , +1 c#0'$ E"en 23 "d&at' at%rate I*4

5%adratic f%nction of /GS no #ore "alid$

6$ )arrier "elocity beco#e a f%nction of nor#al &"ertical' co#ponent E($

Scattering of %rface charge lead to colliion7 %rface #obility

drop$

( ) ( )TGS

n

cGS

Siox

ox

n

x

nn

VVyVV

t

Eeff

+

+

=+

=

1)(.

.1

.1)( 000


10/19

Red%ction in threhold "oltage+$ Long-channel e(preion of /T no #ore "alid$

6$ *epletion region created by *rain &So%rce'-pn 8%nction 9ith b%l: to be

conidered$

;$ Shape of Gate-ind%ced channel i trape!oid &not rectang%lar'

Short )hannel Effect




11/19

Estimation of VTfor Short Channel device



BOQ

,here #nd term is error term due to charge

difference bet,een rectangular and traezoidal deletion region.

The traezoidal bul! deletion channel*region contains charge given by-

The +unction deletion region deths are give by and

,here +unction built*in voltage is

rom the cross*sectional vie, of the +unction deletion layer and deth of the channel

into the bul! ,e can ,rite- ,hich leads to-

and

000 )( TTT VVelshortchannV =

FASiDS

BO NqL

LLQ

+= 2....2.

21

( )DSA

SidD V

Nq

x += 0.

.

.2

0.

.

.2

A

SidS

Nq

x =

=

20

.ln.

i

AD

n

NN

q

kT

( ) ( )222 DjdmdDj Lxxxx ++=+

+ 1

21.

j

dSjS

x

xxL

+ 1

21.

j

dDjD

x

xxL


12/19



Finally fro# and 9e can get the charge

difference in the trape!oidal channel 9hich lead to the

error ter# of threhold "oltage at *rain-So%rce biaia< 0TV


13/19

Shortco#ing of Scaling - II



In Narrow Channel, W is on the order of maximum depletion regionthickness !dm"# Effect: Increase in threshold voltage


14/19

/T IN NARRO? )hannel



The Gate-area o"erlap on the large FO@-area in thefringe$

An additional hallo9 depletion region for#$

For large ?7 thi i negligible$ >%t for co#parable ?&and thin to('7 /T goe %p d%e to thi depletion charge$

beco#e 9here

for an e#pirical para#eter dependent on hape of fringe gi"en by for hape of 5%arter-circ%lar arc$

W

xNq

CV dm

FASiox

T

.22.1

0

=

( )channelnarrowVT 0 00 TT VV +

2

=


15/19

Oth r limitations of small g om try



Two$dimensional electric field vector controls $ cannot %e decoupled#

Complications& e#g#, for V'S VT, potential %arrier controlled %oth %( V'S

and V)S # Increase in V)Slowers the %arrier#

This )rain Induced *arrier +owering )I*+" leads to su%$threshold I)#

I)caused %( )I*+ is expressed as&

where xc

is the sub-threshold channel depth, Dn

is the electron diffusion

coefficientLB

is the length of the barrier andris a reference potential.

( )( )

kT

VBVAq

kT

q

B

cn

D

DSGSr

eeL

nWxqDldsubthreshoI

..

0 ..

+


16/19

unch !hrough

or large /"S deletions region of drain extends

to,ards source.

or small geometry the t,o deletion regions mayeven merge.

This is called 0unch Through2 ,hich can damage thedevice




17/19

I" #O$ES and O%id &r a'(do)n

&n ractice all dimensions cannot be reduceduniformly.

&f tox e.g. is reduced by S3 for very small geometrylocalized non*uniform oxide gro,th called 0in*4oles2

develo.

Another roblem ,ith very thin tox is 05xide

6rea!do,n2




18/19

#ot carri r g n ration

%onstant voltage increased doing and smallgeometry combine to increase electric field.

&ncreased electric field generates electrons and holesat high !inetic energy called 04ot %arrier2.

04ot %arriers2 in+ected into gate*oxide damages oxide7

interface charge distribution.

&*/ characteristics change




19/19

At high /*S and in #all geo#etrie the effect iober"ed$

*a#age i locali!ed on drain end$

Re%lt i red%ction of I* and general

degradation o"er a ti#e$

* piction of #ot carri r *+,+-E



device scaling

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