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Short Channel EfectsShort Channel Efects

in MOSFETsin MOSFETs

Fabio DAgostinoFabio DAgostino

Daniele QuerciaDaniele Quercia

Fall, 2000Fall, 2000

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PresentationPresentation Outline

Short-Channel Devices

Short-Channel Effects (SCE)

The modification of the threshold voltage due to SCE

A numerical example

Simulation: SCE impacts on the threshold voltage

Simualtion: limiting effect of the saturation velocit

Conclusion

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DenitionDenition

What is a short-channel device?

A !"S#ET is considered to $e short %hen the channellength is the same order of magnitude as the depletion-laer %idths (xdD& xdS)

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Short Channel EffectsShort Channel Effects

Five different h!sical henono"ena have to be

considered in short-channel devices#

Drain induced $arrier lo%ering and 'unchthrough

Surface scattering

elocit saturation

mpact ioni*ation

+ot electrons

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Drain-induced barrier loweringDrain-induced barrier lowering

(DI!"(DI!"

The electrons (carriers) in the channel face a potential

$arrier that $loc,s their flo%s

The potential $arrier& in small-geometr !"S#ETs& is

controlled $ a t%o-dimensional electric field vector (inother %ords $ $oth Sand DS)

f the drain voltage is increased the potential $arrierin the channel decreases& leading to

Drain-$nduced %arrier &o'ering (D$%&)

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Drain-induced barrier lowering (DI!" andDrain-induced barrier lowering (DI!" and

PunchthroughPunchthrough

.nder D/0 condiction electrons can flo% $et%een the

source and drain even if S 1 T

The channel current that flo%s in this case is calledsu$threshold current

*unchthrough

The D/0 phenomenon can $e accompanied $ the so-

called punchthrough& that occurs %hen the depletionregion surrounding the drain extends to the source

'unchthrough minimi*ed %ith thinner oxide& larger

su$strate doping (and longer channel2)

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Sur#ace scatteringSur#ace scattering

#or small-geometr !"S#ETs& the electrons mo$ilit in

the channel depends on a t%o-dimensional electric field(+& !)

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Sur#ace scatteringSur#ace scattering

The surface scatteringoccurs %hen electrons areaccelerated to%ard the surface $ the verticalcomponent of the electric field +

The collision of the electrons causes a reduction in themo$ilit

Electrons moves %ith great difficult parallel to the

interface

The average surface mo$ilit is a$out half as muchas that of the $ul, mo$ilit

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$elocit% saturation$elocit% saturation

#or lo% !the electron drift velocit vdein the channel

varies linearl %ith the electric field intensit

As !increases a$ove 3456cm the drift velocit tends

to approach a saturation value of vde(sat)7348cm6s around

!73496cm

The velocit saturation reduces the transconductanceof short-channel devices in the saturation condiction&as the follo%ing formula sho%s:

gm7 Coxvde(sat)

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I&'act ioniationI&'act ioniation

he resence of high longitudinal fields can

accelerate electrons that "a! be able of ioniing .iato"s b! i"acting against the"

;ormall most of the e- are attracted $ the drain& soit is plausi$le a higher concentration of holes near thesource

f the holes concentration on the source is a$le tocreates a voltage drop on the source-su$strate n-p

thene- ma $e in

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)ot electrons)ot electrons

he channel /ot lectronseffect is caused b!

electrons flo'ing in the channel for large 1D.

e- arriving at the Si-Si"?interface %ith enough ,inetic

energ to surmount the surface potential $arrier are

inected into the o+ide

This ma degrade permanentl the C- characteristicsof a !"S#ETs

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The &odication o# theThe &odication o# the

threshold *oltagethreshold *oltagedue to short-channeldue to short-channel

efectsefects

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Modication o# $Modication o# $T)T)due to SCEdue to SCE

3uation giving the threshold voltage atero-bias

( )ox

IFASi

ox

FFBTCqDNq

CVV +

++= 22120

accurate for large !"S transistors

not accurate for short-channel !"S transistorsthe amount of $ul, charge is overestimated

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&arge 45. transistor# the delition isonl! due to the electric fieldcreated b! the gate voltage6

."all-geo"etr! transistor# in addition tothe revious contribution, thedelition charge near n7 regions isinduced b! -n unctions6

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he bul8 deletion charge is s"aller thane+ectedthe threshold voltage e+ression "ust be"odified to account for this reduction#

00)(0 TTchannelshortT VVV =

10# thresholdvoltage shift

10# ero-bias

thresholdvoltage

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We find the follo'ing relationshi#

( ) ( )222DjdmdDj Lxxxx ++=+

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9 and solving for LD

we obtain#

'here

( )

+++= 1

212

222

j

dDjdDjdDdmjjD

x

xxxxxxxxL

( )0

2

+

= DSASi

dD

VqN

x

.i"ilarl!, the length &.can also be found as

follo's#

'here( )0

2

=

A

SidS

qN

x

+ 1

21

j

dSjSx

xxL

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he a"ount of the threshold voltage reduction

1,0due to short-channel effects can be foundas#

++

+= 1

211

21

222

10

j

dS

j

dDj

FASi

ox

Tx

x

x

x

L

xNq

CV

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e consider an n-channel !"S process %ith the follo%ingparameters:

=su$strate doping densit :A;&=gate oxide tic,ness to+; 0 n"&

=oxide-interface fixed charge densit :o+;@6

n addition& %e assume that the channel region is implanted%ith p-tpe impurities(impurit concentration :$; 2

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We obtain 9

VT0

= 0.855V - VT0

;

where ___________

VT0= ( 0.343/L[m] ) * (-0.724 + ( 1 + 2 xdD) _________________

xDd= 0.13 (0.76 + VDS)

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9 and lotting the variation of the thresholdvoltage 'ith the channel lenght

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Vth:T

hreshold

voltage

[V]

(Vth vs. L) @ Vds=1V [-----] Vds=

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Si&ulation+ i&'act o# SCE on the thresholdSi&ulation+ i&'act o# SCE on the threshold

*oltage*oltage

e simulate four n!"S#ETs in parallel& %ith differentchannel lengths and 'idths

All the transistors have the same parameters@ 0EE0 ?

of 'spice is used

#or each transistor %e generate the D-Scharacteristic at DS 7 4=3

The plots sho% ho% devices %ith smaller geometr havehigher drain currents at the same gate-to-sourcevoltage (i=e=& smaller threshold voltages)

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Si&ulation+ i&'act o# SCE on the thresholdSi&ulation+ i&'act o# SCE on the threshold

*oltage*oltage

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Si&ulation+ the li&iting efects o# the saturationSi&ulation+ the li&iting efects o# the saturation

*elocit%*elocit%

e simulate t%o n!"S#ETs in parallel& %ith the samechannel length and %idth

"ne transistor has a limited saturation velocit of vde(sat)

7 2C

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Si&ulation+ the li&iting efects o# the saturationSi&ulation+ the li&iting efects o# the saturation

*elocit%*elocit%

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ConclusionConclusion

.E are governed b! co"le+ h!sical heno"enathat can be "ainl! related to the

$nfluence of both vertical and horiontal

electric field co"onents on the flo'of the electrons in the channel

suall! .E interacts the one 'ith the other

.E should be carefull! considered in order toevaluate their i"act on the general behaviour ofthe device, both for short-ter" and long-ter"

talk (1).ppt

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