Digital Integrated Circuits · 2017. 6. 20. · Leakage Summary •Subthreshold Leakage: •I DS >0 when V GS

Disclaimer: This course was prepared, in its entirety, by Adam Teman. Many materials were copied from sources freely available on the internet. When possible, these sources have been cited;

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Digital Integrated Circuits(83-313)

Lecture 12:

The NanoScaled MOSFETSemester B, 2016-17

Lecturer: Dr. Adam Teman

TAs: Itamar Levi, Robert Giterman

19 June 2017

mailto:[email protected]

Lecture Content

2

Leakage in NanoScaled Transistors

3

Leakage in Nanoscaled Transistors

• Transistors that are supposed to be off actually leak!

4

Main Types of Leakage

5

P-substrate

n+n+

Source Drain

Polysilicon Gate

Oxide

subthreshold

Ga

te

Punchthrough

GS TV V

subI e

1,gate GC

ox

I Vt

GIDL DGI V

DSVDIBL e

Subthreshold Leakage

• When VGS


• Let’s make things easier:

• Remember that:

• And we now defined the threshold voltage according to current:

• So the edge condition requires:

• And we can now calculate subthreshold current as:

7

subGS T

T

V Vn

DS

WI Const e

L

100GS T

T

V Vn

sub

WI nA e

L

100nADS GS TW

I V VL

0

sub 100nATGS T

n

DS V V

W W WI Const e Const

L L L


• An even easier way is to define the subthreshold slope:

• And Ioff, which is defined as the current when VGS=0 is:


8

100

100 10

GS T

T

GS T

V Vn

sub

V VS

WI nA e

L

W

L

100 10TV

Soff

WI nA

L

ln 10 1 2.3dep Tox

CkTS n

q C



• So subthreshold leakage is:

• Exponentially dependent on VGS.

• Exponentially dependent on VT.

• S is the subthreshold swing coefficient.

• Optimally, (Sopt)-1=60 mV/dec

• Realistically (S)-1≈100 mV/dec

9

ln10 0.06TS n

1dep

oxe

Cn

C



Example:

• We want to design a transistor with:

• What is the minimum VT?

10

410on

off

I

I

mV60

decS


Impact of DIBL

• DIBL causes an additional exponential increase

in subthreshold leakage with VDS.

11

90nm technology.

Gate length: 45nm

Intel, T. Ghani et al., IEDM 2003

0 (1 )

GS T DS DS

t t t

V V V V

n n

subI I e e e


Subthreshold Dependence on Temperature

• This is rather complex, as mobility degrades with temperature and

other device values such as flatband voltage are temperature

dependent.

• Altogether, subthreshold leakage rises

exponentially with temperature.

12

GS T

T

V V

subI e

TkT

q


Temperature Inversion

• Standard approach to temperature effect on delay:

• BUT!

• VT decreases by as much as -3mV/°C

13

1

I

T

1pdt T

1T

pd T

V T

t V

pdt T


Gate Leakage

• Two mechanisms:• Direct tunneling (dominant)

• Fowler Nordheim tunneling

• Exponentially Dependent on:• Gate Voltage (VG)

• Oxide Thickness (tox).

• Non-dependent on temperature.

• Much stronger in nMOS than pMOS (higher barrier for holes)

• Minimum tox=1.2nm!!!

14


Gate Leakage

Gate Induced Drain Leakage

• GIDL current flows from the drain to the substrate.

• Caused by high electric field under the gate/drain

overlap, causing e-h pair creation.

• Main phenomena is Band-to-Band Tunneling

15


Gate Leakage

GIDL

Diode Leakage

• JS=10-100pA/μm2 @25°C for 0.25μm CMOS.

• JS doubles for every 9°C

• Much smaller than other leakages in deep sub-micron.

16

DL SI J A


Gate Leakage

GIDL

Diode Leakage

Punchthrough

• As VDS grows, so does the drain depletion region,

and the channel length decreases.

• In severe cases, the source and drain are connected

causing non-controllable leakage current.

17

rjWs

xorjWD

L +VD

rjWs

xorjWD

L++VD

e-


Gate Leakage

GIDL

Diode Leakage

Punchthrough

Leakage Summary• Subthreshold Leakage:

• IDS>0 when VGS0 due to direct tunneling through the oxide.

• Grows with VGB

• Gate Induced Drain Lowering (GIDL):• IDB>0 due to high electric field in the GD overlap region.

• Band to Band tunneling.

• Reverse Biased Diode Leakage• ISB, IDB Due to diffusion and thermal generation.

• Punchthrough:• IDS due to drain and source depletion layers touching.

18


Gate Leakage

GIDL

Diode Leakage

Punchthrough

Latchup

• Latchup is more of a design problem than a leakage current.

• “Thyristors” created by parasitic BJT transistors

can turn on and short VDD and GND.

• This requires power down at the least, and

sometimes causes chip destruction.

19


Gate Leakage

GIDL

Diode Leakage

Punchthrough

Latchup

Secondary Effects and Solutions

20

Problem: Mobility Degradation

• Reminder: we have degraded mobility due to:

• Velocity Saturation

• Surface Scattering

• …and in general, we want more speed!

• There are a number of solutions that are currently

used or are being developed:

• Strained Silicon

• Miller Index

• Different Materials

21

Mobility Degradation

Problem: Surface Scattering

• The mobility at the surface is vertical field (VG) dependent.

• The stronger the field, the more carriers

“hit” the interface and scatter. 0

1 GS TV V


Solution: Strained Silicon

23

Chang, IEDM 2005

Enhanced Channel Mobility


Solution: Improved Channel Materials

24


Solution: Silicon Orientation

25


Problem: Serial Resistance

• The resistance of the Source and Drain areas, especially

with Lightly Doped Drain (LDD),

can have a large impact (>15%)

on transistor conductance

)(1 0

0

tgs

sdsat

dsatdsat

VV

RI

II

0 0dsat dsat dsat s dV V I R R

K. Kuhn, IEDM 2008

Serial Resistance


Solution: Salicides

• Salicide = Self Aligned Silicide

• A low resistance contact is formed

on the diffusion and polysilicon

surface through annealing.

27

Serial Resistance


Source: Wikipedia

Problem: Hot Carrier Effects

• Reminder:• Carriers in strong electric fields jump over the gate energy barrier

and get stuck in the oxide.

• This causes VT to change over time.

• Happens mainly close to the drain

where strong fields exist.

• Solution:• To solve this, Lightly Doped Drains (LDD) are used.

• The N- area reduces the field

gradient near the drain.

• N+ is needed for ohmic contacts.

28

N gradient

n-n+

Serial Resistance


Hot Carriers

Problem: Subthreshold Leakage

• Reminder:• There is a finite number of free carriers in the channel when VGS

Solution: MTCMOS

• A very common solution, already available in most PDKs for

many generations is multi-threshold voltage devices.• High-VT (HVT) devices are slow but have lower leakage

• Low-VT (LVT) devices are fast but very leaky

• Nominal/Standard/Regular-VT (NVT/SVT/RVT) transistors

are best for most operations.

• This approach provides an easy way to trade off

high-performance and low leakage.

• Standard cells are often designed with identical footprints in

order to exchange various VT options seamlessly.

30

Serial Resistance



Hot Carriers

Solution: Body Biasing

• As we have previously seen, the body voltage of the transistor

affects the threshold voltage of the transistor.

• The body voltage of PMOS devices, residing in a shared NWELL

can be set without the need for special process steps.

• Changing the body voltage of NMOS devices requires the use of an

isolated PWELL (IPW) inside a Deep NWELL (DNW) area.

• The effectiveness of body biasing has thoroughly degraded with process

scaling.

• New technologies, such as Fully-depleted Ultra-thin body and buried oxide

Silicon-on-Insulator (FD-SOI or UTBB-SOI) have given this technique “new life”.

31

Serial Resistance



Hot Carriers

Solution: Vertical Dimensions

• A better model of VT, taking into account both DIBL and Roll-Off shows a strong dependence on the transistor’s vertical dimensions:

• tox – gate thickness• Wdep – depletion width• Xj – S/D junction depth

• Reduction of each of these provides a smaller Lmin and other advantages.

• smaller tox means better transconductance.

• smaller Xj means lower S/D capacitance

• smaller Wdep means better drain-channel isolation32

0.4 dL

l

t t long dsV V V e

3d ox dep jl t W X

Serial Resistance



Hot Carriers


• Thinner oxide is the most important

aspect of scaling, as it allows better

gate control.

• Shallow junctions are achieved by:• High Substrate Doping (not wanted)

• Lightly Doped Drains (LDD)

• Metal S/D

• What about Wdep?33

3d ox dep jl t W X

Serial Resistance



Hot Carriers


• Wdep can be reduced by

“Retrograde Doping”• Reduces impurity scattering

(improves mobility).

• Wdep does not vary with VSB.• Causes a linear body effect.

• Reduces punchthrough

34

data model

-2 -1 0 1 2 Vsb (V)

NFET

PFET

Vt 0

Vt0

0.6

-0.2

-0.6

0.4

-0.4

Vt (V)

0.2

Serial Resistance



Hot Carriers

3d ox dep jl t W X

Solution: Multi-gate Transistors

• To gain better control of the channel

(better subthreshold slope) use multiple gates:

35

Double-gate MOSFETFinFET

Serial Resistance



Hot Carriers

Solution: Multi-gate Transistors

36

Serial Resistance



Hot CarriersA. Planar bulk MOSFET

B. SOI MOSFET

C.Tri-gate SOI nanowire

MOSFET

D.Tri-gate Bulk FinFET

E. Tri-gate SOI FinFET

F. Pi-gate SOI nanowire MOSFET

G.Omega-gate SOI nanowire

MOSFET

H.Horizontal Gate-all-around (GAA)

nanowire MOSFET

I. Vertical GAA nanowire MOSFET

Ferain, Nature 2011

Solution: Silicon-On-Insulator

• Silicon on Insulator (SOI) is a better

(though more expensive) way to make transistors:• Fixed depletion widths (Wdep, Xj) – lower Roll-Off/DIBL

• No body effect.

• No Punchthrough.

• Junction to substrate parasitic capacitance small.

• No latch-up between NMOS and PMOS (no substrate)

37

Serial Resistance



Hot Carriers

Solution: Silicon-On-Insulator

• Lower diffusion capacitance:

• Raised Source/Drain for reduced resistivity.

38

Serial Resistance



Hot Carriers

Problem: Gate Leakage

• Reminder:

• Gate leakage is exponentially dependent on tox.

• A thin oxide reduces Roll-Off.

• A thin oxide provides higher transconductance (Cox)

• A thin oxide reduces the subthreshold swing:

• Major Problem:

• Oxide thickness reached 1.2nm (5 atomic layers) in 65nm.

• Gate leakage is intolerable under tox=1.5nm

• The end of scaling???

39

1

ln10TS

n1

dep

oxe

Cn

C

Serial Resistance


Gate Leakage


Hot Carriers

Solution: High-K Dielectrics

• Higher K (ε) means Cox is increased, therefore:• Transconductance is higher.

• n=1+CD/Cox is lower Better gate control.

• So we can use thicker gates to eliminate tunneling

without losing control or drive strength.

40

Serial Resistance


Gate Leakage


Hot Carriers

Solution: High-K Dielectrics

• HfO2 has a relative dielectric constant (k) of ~24, six times large than that of SiO2. • For the same EOT, the HfO2 film presents a much thicker

(albeit a lower) tunneling barrier to the electrons and holes.

• Toxe can be further reduced by introducing metal-gate technology since the poly-depletion effect is eliminated.

• The difficulties of high-k dielectrics:• Chemical reactions between them and the silicon

substrate and gate

• Lower surface mobility than the Si/SiO2 system

• Too low a VT for pMOS(as if there is positive charge in the high-k dielectric).

• Long-term reliability41

Serial Resistance


Gate Leakage


Hot Carriers

Problem: Punchthrough Currents

• Reminder:

• Drain and Source depletion regions “connect”

to each other deep underneath the channel.

• Solutions:

• Halo Implants

• LDD

• Retrograde Doping

• SOI

42

Serial Resistance


Gate Leakage

Punchthrough


Hot Carriers

Solution: Halo Implants

• p+ implants suppress the

source/drain depletion regions.

• However, the also cause the

Reverse Short Channel Effect.

43

Serial Resistance


Gate Leakage

Punchthrough


Hot Carriers

Problem: Latch Up

• Reminder:

• A Thyristor is created because of voltage drop

over the substrate (or well).

• In general, cases of forward biased diodes

are often called latchup.

• Solutions:

• Body Taps.

• SOI.

44

Serial Resistance


Gate Leakage

Punchthrough

Latchup


Hot Carriers

Further Reading• J. Rabaey, “Digital Integrated Circuits” 2003, Chapters 2.5, 3.3-3.5

• Weste, Harris “CMOS VLSI Design”, Chapter 7

• C. Hu, “Modern Semiconductor Devices for Integrated Circuits”, 2010, Chapters 4-7 http://www.eecs.berkeley.edu/~hu/Book-Chapters-and-Lecture-Slides-download.html

• Tzividis, et al. “Operation and Modeling of MOS Transistor” New York Press 2011. Chapters 1-5

• E. Alon, Berkeley EE-141, Lecture 9 (Fall 2009) http://bwrc.eecs.berkeley.edu/classes/icdesign/ee141_f09/

• M. Alam, Purdue ECE-606 – lectures 32-38 (2009) http://nanohub.org/resources/5749

• A. B. Bhattacharyya “Compact MOSFET models for VLSI design”, 2009,

• T. Sakurai, “Alpha Power-Law MOS Model” – JSSC Newsletter Oct 2004

• Managing Process Variation in Intel’s 45nm CMOS Technology, Intel Technology Journal, Volume 12, Issue 2, 2008

• Berkeley “BSIM 4.6.4 User’s Manual” http://www-device.eecs.berkeley.edu/~bsim/BSIM4/BSIM464/BSIM464_Manual.pdf

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Digital Integrated Circuits · 2017. 6. 20. · Leakage Summary •Subthreshold Leakage: •I DS >0 when V GS