Digital Integrated Circuit Design The Devices Digital Integrated Circuit Design Andrea Bonfanti DEIB...

Digital Integrated Circuit Design The Devices

The DevicesThe Devices

Digital Integrated Circuit Digital Integrated Circuit DesignDesignAndrea BonfantiDEIBVia Golgi 40, Milano

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Digital Integrated Circuit Design The Devices

Aims of this chapterAims of this chapter

Present intuitive understanding of device operation

Introduction of basic device equations Introduction of models for manual analysis Analysis of secondary and deep-sub-micron

effects Future trends

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Digital Integrated Circuit Design The Devices

The DiodeThe Diode

n

p

p

n

B A SiO2Al

A

B

Al

A

B

Cross-section of pn-junction in an IC process

One-dimensionalrepresentation diode symbol

Mostly occurring as parasitic element in Digital ICs

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Digital Integrated Circuit Design The Devices

Depletion RegionDepletion Regionhole diffusion

electron diffusion

p n

hole driftelectron drift

ChargeDensity

Distancex+

-

ElectricalxField

x

PotentialV

W2-W1

(a) Current flow.

(b) Charge density.

(c) Electric field.

(d) Electrostaticpotential.

4

Digital Integrated Circuit Design The Devices

Diode CurrentDiode Current

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Digital Integrated Circuit Design The Devices

Forward BiasForward Bias

x

pn0

np0

-W1 W20p n

(W2)

n-regionp-region

Lp

diffusion

Typically avoided in Digital ICs6

Digital Integrated Circuit Design The Devices

Reverse BiasReverse Bias

x

pn0

np0

-W1 W20n-regionp-region

diffusion

The Dominant Operation Mode7

Digital Integrated Circuit Design The Devices

Models for Manual AnalysisModels for Manual Analysis

VD

ID = IS(eVD/T – 1)+

–

VD

+

–

+

–VDon

ID

(a) Ideal diode model (b) First-order diode model

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Digital Integrated Circuit Design The Devices

Junction CapacitanceJunction Capacitance

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Digital Integrated Circuit Design The Devices

Diffusion CapacitanceDiffusion Capacitance

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Digital Integrated Circuit Design The Devices

Secondary EffectsSecondary Effects

–25.0 –15.0 –5.0 5.0

VD (V)

–0.1

I D (A

)

0.1

0

0

Avalanche Breakdown

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Digital Integrated Circuit Design The Devices

Diode ModelDiode Model

ID

RS

CD

+

-

VD

12

Digital Integrated Circuit Design The Devices

SPICE ParametersSPICE Parameters

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Digital Integrated Circuit Design The Devices

What is a Transistor?What is a Transistor?

VGS VT

RonS D

A Switch!

|VGS|

An MOS Transistor

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Digital Integrated Circuit Design The Devices

The MOS TransistorThe MOS Transistor

Polysilicon Aluminum

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Digital Integrated Circuit Design The Devices

MOS Transistors -MOS Transistors -Types and SymbolsTypes and Symbols

D

S

G

D

S

G

G

S

D D

S

G

NMOS Enhancement NMOS

PMOS

Depletion

Enhancement

B

NMOS withBulk Contact

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Digital Integrated Circuit Design The Devices

Threshold Voltage: ConceptThreshold Voltage: Concept

n+n+

p-substrate

DSG

B

VGS

+

-

Depletion

Region

n-channel

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Digital Integrated Circuit Design The Devices

The Threshold VoltageThe Threshold Voltage

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Digital Integrated Circuit Design The Devices

The Body EffectThe Body Effect

-2.5 -2 -1.5 -1 -0.5 00.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

VBS

(V)

VT (

V)

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Digital Integrated Circuit Design The Devices

Current-Voltage RelationsCurrent-Voltage RelationsA good ol’ transistorA good ol’ transistor

QuadraticRelationship

0 0.5 1 1.5 2 2.50

1

2

3

4

5

6x 10

-4

VDS (V)

I D (

A)

VGS= 2.5 V

VGS= 2.0 V

VGS= 1.5 V

VGS= 1.0 V

Resistive Saturation

VDS = VGS - VT

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Digital Integrated Circuit Design The Devices

Transistor in LinearTransistor in Linear

n+n+

p-substrate

D

SG

B

VGS

xL

V(x) +–

VDS

ID

MOS transistor and its bias conditions21

Digital Integrated Circuit Design The Devices

Transistor in SaturationTransistor in Saturation

n+n+

S

G

VGS

D

VDS > VGS - VT

VGS - VT+-

Pinch-off

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Digital Integrated Circuit Design The Devices

Current-Voltage RelationsCurrent-Voltage RelationsLong-Channel DeviceLong-Channel Device

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Digital Integrated Circuit Design The Devices

A model for manual analysisA model for manual analysis

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Digital Integrated Circuit Design The Devices

Current-Voltage RelationsCurrent-Voltage RelationsThe Deep-Submicron EraThe Deep-Submicron Era

LinearRelationship

-4

VDS (V)0 0.5 1 1.5 2 2.5

0

0.5

1

1.5

2

2.5x 10

I D (

A)

VGS= 2.5 V

VGS= 2.0 V

VGS= 1.5 V

VGS= 1.0 V

Early Saturation

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Digital Integrated Circuit Design The Devices

Velocity SaturationVelocity Saturation

(V/µm)c = 1.5

n

(m/s

)

sat = 105

Constant mobility (slope = µ)

Constant velocity

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Digital Integrated Circuit Design The Devices

PerspectivePerspective

IDLong-channel device

Short-channel device

VDSVDSAT VGS - VT

VGS = VDD

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Digital Integrated Circuit Design The Devices

IIDD versus V versus VGSGS

0 0.5 1 1.5 2 2.50

1

2

3

4

5

6x 10

-4

VGS (V)

I D (

A)

0 0.5 1 1.5 2 2.50

0.5

1

1.5

2

2.5x 10

-4

VGS (V)

I D (

A)

quadratic

quadratic

linear

Long Channel Short Channel

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Digital Integrated Circuit Design The Devices

IIDD versus V versus VDSDS

-4

VDS (V)0 0.5 1 1.5 2 2.5

0

0.5

1

1.5

2

2.5x 10

I D (

A)

VGS= 2.5 V

VGS= 2.0 V

VGS= 1.5 V

VGS= 1.0 V

0 0.5 1 1.5 2 2.50

1

2

3

4

5

6x 10

-4

VDS (V)

I D (

A)

VGS= 2.5 V

VGS= 2.0 V

VGS= 1.5 V

VGS= 1.0 V

ResistiveSaturation

VDS = VGS - VT

Long Channel Short Channel

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Digital Integrated Circuit Design The Devices

A unified modelA unified modelfor manual analysisfor manual analysis

S D

G

B

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Digital Integrated Circuit Design The Devices

Simple Model versus SPICE Simple Model versus SPICE

0 0.5 1 1.5 2 2.50

0.5

1

1.5

2

2.5x 10

-4

VDS

(V)

I D (

A)

VelocitySaturated

Linear

Saturated

VDSAT=VGT

VDS=VDSAT

VDS=VGT

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Digital Integrated Circuit Design The Devices

A PMOS TransistorA PMOS Transistor

-2.5 -2 -1.5 -1 -0.5 0-1

-0.8

-0.6

-0.4

-0.2

0x 10

-4

VDS (V)

I D (

A)

Assume all variablesnegative!

VGS = -1.0V

VGS = -1.5V

VGS = -2.0V

VGS = -2.5V

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Digital Integrated Circuit Design The Devices

Transistor Model Transistor Model for Manual Analysisfor Manual Analysis

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Digital Integrated Circuit Design The Devices

The Transistor as a SwitchThe Transistor as a Switch

VGS VT

RonS D

ID

VDS

VGS = VD D

VDD/2 VDD

R0

Rmid

ID

VDS

VGS = VD D

VDD/2 VDD

R0

Rmid

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Digital Integrated Circuit Design The Devices

The Transistor as a SwitchThe Transistor as a Switch

0.5 1 1.5 2 2.50

1

2

3

4

5

6

7x 10

5

VDD

(V)

Req

(O

hm)

35

Digital Integrated Circuit Design The Devices

The Transistor as a SwitchThe Transistor as a Switch

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Digital Integrated Circuit Design The Devices

MOS CapacitancesMOS CapacitancesDynamic BehaviorDynamic Behavior

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Digital Integrated Circuit Design The Devices

Dynamic Behavior of MOS TransistorDynamic Behavior of MOS Transistor

DS

G

B

CGDCGS

CSB CDBCGB

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Digital Integrated Circuit Design The Devices

The Gate CapacitanceThe Gate Capacitance

tox

n+ n+

Cross section

L

Gate oxide

xd xd

L d

Polysilicon gate

Top view

Gate-bulkoverlap

Source

n+

Drain

n+W

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Digital Integrated Circuit Design The Devices

Gate CapacitanceGate Capacitance

S D

G

CGC

S D

G

CGC

S D

G

CGC

Cut-off Resistive Saturation

Most important regions in digital design: saturation and cut-off

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<

Digital Integrated Circuit Design The Devices

Gate CapacitanceGate Capacitance

WLCox

WLCox

2

2WLCox

3

CGC

CGCS

VDS /(VGS-VT)

CGCD

0 1

CGC

CGCS = CGCDCGC B

WLCox

WLCox

2

VGS

Capacitance as a function of VGS(with VDS = 0)

Capacitance as a function of the degree of saturation

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0V

Digital Integrated Circuit Design The Devices

Measuring the Gate CapMeasuring the Gate Cap

2 1.52 12 0.5 0

3

4

5

6

7

8

9

103 102 16

2

VGS (V)

VGS

Gate

Ca

paci

tan

ce (

F)

0.5 1 1.5 22 2

I

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Digital Integrated Circuit Design The Devices

Diffusion CapacitanceDiffusion Capacitance

Bottom

Side wall

Side wallChannel

SourceND

Channel-stop implant NA1

SubstrateNA

W

xj

LS

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Digital Integrated Circuit Design The Devices

Junction CapacitanceJunction Capacitance

44

Digital Integrated Circuit Design The Devices

Linearizing the Junction CapacitanceLinearizing the Junction Capacitance

Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces equal charge over voltage swing of interest

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Digital Integrated Circuit Design The Devices

Capacitances in 0.25 Capacitances in 0.25 m CMOS m CMOS processprocess

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Digital Integrated Circuit Design The Devices

The Sub-Micron MOS TransistorThe Sub-Micron MOS Transistor

Threshold Variations Subthreshold Conduction Parasitic Resistances

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Digital Integrated Circuit Design The Devices

Threshold VariationsThreshold Variations

VT

L

Long-channel threshold Low VDS threshold

Threshold as a function of the length (for low VDS)

Drain-induced barrier lowering (for low L)

VDS

VT

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Digital Integrated Circuit Design The Devices

Sub-Threshold ConductionSub-Threshold Conduction

0 0.5 1 1.5 2 2.510

-12

10-10

10-8

10-6

10-4

10-2

VGS (V)

I D (

A)

VT

Linear

Exponential

Quadratic

Typical values for S:60 .. 100 mV/decade

The Slope Factor

ox

DnkT

qV

D C

CneII

GS

1 ,~ 0

S is VGS for ID2/ID1 =10

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Digital Integrated Circuit Design The Devices

Sub-Threshold Sub-Threshold IIDD vs vs VVGSGS

VDS from 0 to 0.5V

kT

qV

nkT

qV

D

DSGS

eeII 10

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Digital Integrated Circuit Design The Devices

Sub-Threshold Sub-Threshold IIDD vs vs VVDSDS

DSkT

qV

nkT

qV

D VeeIIDSGS

110

VGS from 0 to 0.3V

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Digital Integrated Circuit Design The Devices

Summary of MOSFET Operating Summary of MOSFET Operating RegionsRegions

Strong Inversion VGS > VT

Linear (Resistive) VDS < VDSAT

Saturated (Constant Current) VDS VDSAT

Weak Inversion (Sub-Threshold) VGS VT

Exponential in VGS with linear VDS dependence

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Digital Integrated Circuit Design The Devices

Parasitic ResistancesParasitic Resistances

W

LD

Drain

Draincontact

Polysilicon gate

DS

G

RS RD

VGS,eff

53

Digital Integrated Circuit Design The Devices

Latch-upLatch-up

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Equivalent model

Digital Integrated Circuit Design The Devices

Future PerspectivesFuture Perspectives

25 nm FINFET MOS transistor

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