EE141 © Digital Integrated Circuits 2nd Devices 1 Digital Integrated Circuits A Design Perspective The Devices Jan M. Rabaey Anantha Chandrakasan Borivoje

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© Digital Integrated Circuits2nd Devices

Digital Integrated Digital Integrated CircuitsCircuitsA Design PerspectiveA Design Perspective

The DevicesThe Devices

Jan M. RabaeyAnantha ChandrakasanBorivoje Nikolic

July 30, 2002

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Goal of this chapterGoal of this chapter

Present intuitive understanding of device operation

Introduction of basic device equations Introduction of models for manual

analysis Introduction of models for SPICE

simulation Analysis of secondary and deep-sub-

micron effects Future trends

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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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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.

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Diode CurrentDiode Current

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Forward BiasForward Bias

x

pn0

np0

-W1 W20p n

(W2)

n-regionp-region

Lp

diffusion

Typically avoided in Digital ICs

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Reverse BiasReverse Bias

x

pn0

np0

-W1 W20n-regionp-region

diffusion

The Dominant Operation Mode

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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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Junction CapacitanceJunction Capacitance

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What is a Transistor?What is a Transistor?

VGS VT

RonS D

A Switch!

|VGS|

An MOS Transistor

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The MOS TransistorThe MOS Transistor

Polysilicon Aluminum

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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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Threshold Voltage: ConceptThreshold Voltage: Concept

n+n+

p-substrate

DSG

B

VGS

+

-

Depletion

Region

n-channel

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The Threshold VoltageThe Threshold Voltage

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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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Transistor in LinearTransistor in Linear

n+n+

p-substrate

D

SG

B

VGS

xL

V(x) +–

VDS

ID

MOS transistor and its bias conditions

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Transistor in SaturationTransistor in Saturation

n+n+

S

G

VGS

D

VDS > VGS - VT

VGS - VT+-

Pinch-off

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A model for manual analysisA model for manual analysis

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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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Process DataProcess Data

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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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Velocity SaturationVelocity Saturation

(V/µm)c = 1.5

n

(m/s

)

sat = 105

Constant mobility (slope = µ)

Constant velocity

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Velocity Saturation (Computation)Velocity Saturation (Computation)

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PerspectivePerspective

IDLong-channel device

Short-channel device

VDSVDSAT VGS - VT

VGS = VDD

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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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A unified modelA unified modelfor manual analysisfor manual analysis

S D

G

B

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Transistor Model Transistor Model for Manual Analysisfor Manual Analysis

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The Sub-Micron MOS TransistorThe Sub-Micron MOS Transistor

Threshold Variations Subthreshold Conduction Parasitic Resistances

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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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The MOS TransistorThe MOS Transistor

Polysilicon Aluminum

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MOSFET Capacitance ModelMOSFET Capacitance Model

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MOSFET Gate CapacitanceMOSFET Gate Capacitance

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MOSFET DB and SB CapacitanceMOSFET DB and SB Capacitance

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Junction CapacitanceJunction Capacitance

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More Process DataMore Process Data

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Problems (2)Problems (2)

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Latch-upLatch-up

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EE141 © Digital Integrated Circuits 2nd Devices 1 Digital Integrated Circuits A Design Perspective The Devices Jan M. Rabaey Anantha Chandrakasan Borivoje