chapter 5-1 short channel effects.pdf

8/11/2019 Chapter 5-1 short channel effects.pdf

http://slidepdf.com/reader/full/chapter-5-1-short-channel-effectspdf 1/32

Chapter 4. Short Channel MOSFET

•

Small Geometry Effects

•

Short Channel, Narrow Width Effects

•

VT

roll-off, DIBL, Effective Mobility for short channel

device•

Velocity Saturation, Id,sat

& Vd,sat

for short channel device

•

I thank Dr Lee Sungjoo and A/P Zhu Chun Xiang for

providing some of the slides in this module.



Small Geometry Effects

Short Channel EffectReverse short channel effect

Narrow Width Effect

Reverse narrow width effect



Short Channel Effect

Short channel transistor : channel length is

comparable to depth of source and drain junctions and the gate depletion depth

Short Channel Effects

-

Threshold voltage roll-off

-

DIBL (Drain Induced Barrier Lowering)

-

Carrier Velocity saturation

- Mobility degradation



Charge Sharing Model

Even with Vgs

=0, part of channel is already depleted. If there was no N+region, the gate depletion will look like the one with green dashes.

The pn

junction theory we know and MOS capacitor theorydeveloped earlier reasonably deal with each of these individually.

The main issue is how to treat the overlap region where both drainand gate electric field are present.

Let us see how short channel effects start to surface andsimple basis behind them.

In absence of the gate, N+P junction depletion nearsource mostly is in p substrate as shown by black dashes

N+

Drain Depletion

Gate

Gate

Gate

Gate

Overlap region




At the top corner of the overlap region, the gate field will dominate and itsinfluence will reduce as the depth increases. The drain field behaves the sameaway from the junction and will progressively dominate the gate field awayfrom the interface.

Hence in this simple picture, the light blue triangle of the overlap region iscontrolled by the drain and white part by the gate.

Clearly, the gate depleted bulk charge region shape is no longer

rectangular and the amount of charge is reduced compared to MOScapacitor due to presence of drain.

As substrate doping is uniform, the gate field decreasesaway from the oxide interface linearly. The drain fieldpeaks at the junction and decreases linearly away fromthe junction.

Gate

Gate

Gate

Gate

Overlap regionN+

Drain Depletion




With source and drain both included with larger reverse bias at the drainand more realistic junction shape, we get the representation above.

Clearly, the reduction in the bulk charge due to different gate depletionshape, Qb

( = Qb

– Q’b

) is negligible for long channel MOSFET. For ashort channel MOSFET this will be significant compared to that of MOScapacitor and a smaller Vg

is required to induce the same amount ofinversion charge. VT

drops.




•

Charge sharing models

account for the reduction

in VT

through the sharing of the channel depletionregion charge between the gate and source/drain

(s/d) junctions. (Channel depletion region is then

geometrically divided into two parts: one associatedwith the gate and the other associated with the s/d

junctions as described earlier.)

•

The accuracy of the models obviously is dependent

on how Qb

is geometrically divided to get Q’b



Yau’s

Model

Basic assumptions:

-

uniformly doped substrate with Nb

and Vds

(= 0.1V) is small.-

S/D junction sidewalls are cylindrical in shape with radius X j.

-

charges at the S/D end of the channel are shared equally between the

gate & the S/D junctions with hashed region attributed to the S/D,

resulting in a trapezoidal shape for the gate controlled depletion charge.- Xsd = Xdd = Xdm 2 ,b

th fb f l b A dmox

QV V F Q qN X

C



Charge Sharing Factor

-

Fraction of the total charge in thechannel that is attributed to thegate ( < 1 ). For long channel

device, this factor is 1.

1

211

j

dm j

l X

X

L

X F

Ideal charge (Qb)

Actual charge (Q’b)=

ox

b sb f f fbth

j

dm j

ox

b

ox

bththth

C

ΔQ

-V V V

X

X

L

X

C

Q

C

QV V V

)2(2channel)short(

12

1)channelshort()channel long(

off;-rollVof amountThe th



VT

roll-off

As channel length decreases, threshold voltagedecreases



Charge Sharing

In order to avoid charge sharing

-

Minimize 2-D effect; Fl 1, Q’b

Qb

-

Give gate more control over depletion charge1)

Tox

: Cox

makes term negligible

2)

X j

: shallow junction, reduces term.

3)

Nb

: to minimize spreading of lateral electric field

But, trade-off of each solution

Vds

exacerbate the charge sharing effect ;

Vds

: Xdd

worse short channel effect as seen

from slide 6.

j

L

b

ox

ΔQ

C



Reverse Short Channel Effect

•

In sub-250nm technologies, VT

initially increases with decreasing channel

length (VT

roll-up)• After VT reaches a maximum value, it then declines as channel lengths are

further decreased (VT roll-off ) as anticipated from standard theory. Why?



Enhanced lateral dopant diffusion model

Uniformly doped channel region is rare in MOS technology.In particular, in short channel technologies, there is a halosubstrate implant done after the gate formation whichintroduces higher doping near gate edges. These edgesdominate in short L raising VT

.

In MOS process, poly gate re-oxidationis done to cure damage and form screenoxide on S/D as shown. Re-oxidationinduce GGO (Graded Gate Oxide),bird’s beak

During re-oxidation, OED (OxideEnhanced Diffusion) of Boron happensnear channel region due to the lateraldiffusion of Si interstitials higherconcentration of B at channel edges

VT increases as L decreases.



DIBL (Drain Induced Barrier Lowering)

The electrons from source areinjected into the channel bysurmounting surface potentialbarrier. Long channel: Barrierheight is not affected by Vds.There is long flat region.

Short channel: Field linesfrom drain penetrate thechannel. Increased Vds

reduces the barrier height atSource. (dashed lines)

DIBL results in the increase ofthe sub-threshold current

lower Vth

.



DIBL

To reduce DIBL effect, need to give gate morecontrol of ΦB

1) Tox

2) Nb

3) X j

4) L

0

0 0 1

0 1

Reduction in threshold voltage due to DIBL( )

DIBL parameter, empirical expression of which is

( )

where , and are constants that are used to better

fit the m

th ds th ds

si sbm

ox

V V V V

is

V

C L

m

odel for the geometry dependence of the DIBL effect



DIBL

Long channel: sub-thresholdcurrent is independent ofVds

.

Short channel: sub-

threshold current isincreased by Vds

at a givegate voltage but no change

in S.S. Hence there is shiftin Vth

at higher Vds

.

Even shorter channel: S.S isincreased surfacepotential is controlled moreby drain than by gate.

Punch-through : depletion of drain junction is punchedthrough to the depletion region of source. In this case, the

MOSFET is on at zero gate voltage and there is no gatecontrol. This normally occurs below surface.



Ctap Cfox

Narrow Width Effect

Edges of gate electrode are over thick field oxide, causinga small depletion region

Gate induced fringing field around the edges introduces an

extra depletion charge, ΔQw

Vg must support this additional depletion region charge

We normally focus on the

channel current crosssection of MOSFET. The

cross section of MOSFET in

the width direction in 250nm

or Older technologies is

shown. The isolation field

oxide encroaches in the

channel area giving bird’s

beak. There is also a

channel stop implant under

field oxide to raise field MOS

threshold voltage.



Narrow Width Effect

1.25 x 1016 cm -3

1.56 x 1016 cm -3

1.71 x 1016 cm -3

ox

b

T

T

th

ox

w sb f f fb

th

C

Q

C

Q

ΔV

C

ΔQ )V φ( γφV

V

22

)channelnarrow(

•

QT

(total gate controlled charge) = Qb

(in

channel depletion region) + 2Qtap

(in the

tapered region) + 2Qfox

(in field oxide

depletion region),

•

CT

= Cox

+ 2n1

Ctap

+ 2

n2

Cfox

where

n1

, n2

account for the edge fringing field

and the potential difference between the

surface potential in the channel and thepotential under the tapered and thick field

oxide regions.

•

QW

has a very strong dependence on

isolation process such as field oxidegeometry, lateral diffusion of channel

stop dopant, and so on.



Small Geometry Effect

Both L and W are of the same order of magnitudeas Xdm

Combined effect of short channel and narrowwidth effects

Estimate Vt

by superposing short channel and

narrow width effects

ox

w

ox

l sb f f fbth

ox

w

ox

l wthl thth

C

Q

C

QV V V

C

Q

C

QV V V

22

,,



Channel Mobility

Carrier mobility in MOSFET channel is lower thanbulk silicon due to the scattering of carriers inchannel and at interface.

(1): Inversion charges,

(2): Ionized impurity charge,

(3): Fixed oxide charge,(4): Interface state charge



Channel Mobility-Scattering Mechanisms

Phonon Scattering (Lattice scattering), μ ph

-

Due to the various modes of lattice vibration including surface

acousticphonons and optical phonons.

-

As temperature increases, the lattice atoms vibrate more and hence thiscomponent tends to become more dominant.

-

Dominant at room temperature and above

-

Actual measured exponent from mobility measurement is not -1.5, but

this component of mobility does decrease with temperature.

Coulomb Scattering, μ c

-

Due to charge centers, including bulk and other charges: Qf

, Qit

, Qb

-

The scattering increases as substrate doping concentration or oxidecharge increases

-

Important for lightly inverted surface (low field), and less effective forheavily inverted surface due to screening of ions by the inversion layer.

This screening tends to raise mobility at low field in the beginning.- Dominant at low temperature

2

3

T ph



Surface Roughness Scattering, μ sr

-

Due to rough interface

-

Important under strong inversion at higher effective field.-

Almost independent of temperature, strong dependence on process

technique.

-

With higher gate field, probability of interface scattering increases.

-

At low temperature, 2 eff sr

Channel Mobility-Scattering Mechanisms



Channel Mobility

1111

;rule sn'Matthiesse

sr c pheff

μ e f f

Eeff

μc

μph

μsr μeff

as , , , ,

?

eff b sb ox g N T V T V

Why suchdepedenceisobserved



Channel Effective Mobility and Effective Field

i

i

x

xn

eff dx xn

dx xn

0

0

)(

)(

eff

: average value weighted by thecarrier concentration in inversion layer

ξeff

or Eeff

: average electric fieldperpendicular to the Si-SiO2

interfaceexperienced by the carrier in the channel. Wayof calculation will be discussed in later slides.



e f f

( c m 2 / V . s

e c )

Channel Mobility



Channel Mobility-Effective Field

3

1 ;holefor

electron,for2

1

2

1 field;average

0

0

00

n D

si

eff

n D

si

eff

si

D

si

n D

QQ

QQ

QQQ

x

QD

Qn

ρ

ξx

si

n D QQ

0

si

DQ 0

x

The factor of 3 is entirely empirical meaning without any physical reasoning

We use inversion charge Qn

and the depletion

charge QD

to calculate average effective field.Qn

is very shallow and the field for this depth is

linear given by the first term assuming uniform

charge density as shown. Similarly the second

term is from depletion charge.



Channel Mobility-Simplified Mobility model

00.3

1 2

1 2

The model above is too complex as it requires detailed calculation

of each component. From the universal curves, surface mobility is

1

where and are fitting parameter,

m 2

s meff eff

for electron, m 1 for hole

0

For the circuit simulation, even more simplified model is used asthe gate voltage determines effective field. SPICE model is

.1 ( ) s gs th

where is fitting parameter V V



Temperature Effects

In practice devices get hot due to Joule heating.

Therefore normally we are interested in increasing T bahaviour.

1. Effective Mobility:

The phonon scattering component decides the strong temperature de

-k

pendence

of the effective mobility as below.

T , ( ) , ( 1.5 ~ 2)

eff

k

eff eff ref ref

T : μ

T μ μ T k

T



Temperature Effects

2. Threshold Voltage

( Si becomes more intrinsic,

meaning less doping, easy to control)2 2

2

12

ln , exp( )2

T

f A si bT MS b

ox ox

b A siT

ox b

g Ab i

i

T : V

Q qN ε ( )V Φ

C C

d qN εdV

dT dT C

E kT N n

q n kT

,

0.5T dV mV dT C



Temperature Effects

3. Subthreshold Swing- Dependence is direct as perthe equation below.

. . (more diffusion occur as T )

. . ln10 1 D

ox

T : S S

kT C S S

q C

4. Drain Current- Here two terms contribute. Asthe mobility depedence is very strong, it controls overall

dependence.

D eff G T

eff D

T D

D

I μ V -V

T : μ , I

T : V , I

T : total I



5. Junction Leakage is determined by the generation in the depletion

regions as given below. Here W is the depletion width and is

the lifetime.2i

g i g qn W J , T : n : J τ

Temperature Effects

chapter 5-1 short channel effects.pdf

Documents