Dispositivi e modelli 1

Progettazione di circuiti e sistemi VLSI

Anno Accademico 2010-2011

Lezione 3

16.3.2012

Dispositivi e modelli

Dispositivi e modelli 2

Goal of this chapter• Presents 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

Dispositivi e modelli 3

The 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

Dispositivi e modelli 4

Depletion 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.

Dispositivi e modelli 5

Forward Bias

x

pn0

np0

-W1 W20

p n(W

2)

n-regionp-region

Lp

diffusion

Typically avoided in Digital ICs

Dispositivi e modelli 6

Reverse Bias

x

pn0

np0

-W1 W20n-regionp-region

diffusion

The Dominant Operation Mode

Dispositivi e modelli 7

Diode Current

Dispositivi e modelli 8

Models for Manual Analysis

VD

ID = IS(eVD/T – 1)+

–

VD

+

–

+

–VDon

ID

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

Dispositivi e modelli 9

Junction Capacitance

Dispositivi e modelli 10

Diode Model

ID

RS

CD

+

-

VD

Dispositivi e modelli 11

SPICE Parameters

Dispositivi e modelli 12

What is a Transistor?

VGS VT

RonS D

A Switch!

|VGS|

An MOS Transistor

Dispositivi e modelli 13

The MOS Transistor

Dispositivi e modelli 14

MOS Transistors -Types and Symbols

D

S

G

D

S

G

G

S

D D

S

G

NMOS Enhancement NMOS

PMOS

Depletion

Enhancement

B

NMOS withBulk Contact

Dispositivi e modelli 15

Threshold Voltage: Concept

n+n+

p-substrate

DSG

B

VGS

+

-

DepletionRegion

n-channel

Dispositivi e modelli 16

The Threshold Voltage

Dispositivi e modelli 17

The 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

)

Dispositivi e modelli 18

Current-Voltage RelationsA good ol’ transistor

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

Dispositivi e modelli 19

Transistor in Linear

n+n+

p-substrate

D

SG

B

VGS

xL

V(x) +–

VDS

ID

MOS transistor and its bias conditions

Dispositivi e modelli 20

Transistor in Saturation

n+n+

S

G

VGS

D

VDS > VGS - VT

VGS - VT+-

Pinch-off

Dispositivi e modelli 21

Current-Voltage RelationsLong-Channel Device

Dispositivi e modelli 22

A model for manual analysis

Dispositivi e modelli 23

Current-Voltage RelationsThe 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

Dispositivi e modelli 24

Velocity Saturation

(V/µm)c = 1.5

un

(m/s

)

usat = 105

Constant mobility (slope = µ)

Constant velocity

Dispositivi e modelli 25

Perspective

IDLong-channel device

Short-channel device

VDSVDSAT VGS - VT

VGS = VDD

Dispositivi e modelli 26

ID versus VGS

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

Dispositivi e modelli 27

ID versus VDS

-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

Dispositivi e modelli 28

A unified modelfor manual analysis

S D

G

B

Dispositivi e modelli 29

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

Dispositivi e modelli 30

A 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

Dispositivi e modelli 31

Transistor Model for Manual Analysis

Dispositivi e modelli 32

The Transistor as a Switch

VGS VT

RonS D

ID

VDS

VGS = VD D

VDD/2 VDD

R0

Rmid

Dispositivi e modelli 33

The Transistor as a Switch

0.5 1 1.5 2 2.50

1

2

3

4

5

6

7x 10

5

VDD (V)

Req

(Ohm

)

Dispositivi e modelli 34

The Transistor as a Switch

Dispositivi e modelli 35

MOS CapacitancesDynamic Behavior

Dispositivi e modelli 36

Dynamic Behavior of MOS Transistor

DS

G

B

CGDCGS

CSB CDBCGB

Dispositivi e modelli 37

The 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

Dispositivi e modelli 38

Gate Capacitance

S D

G

CGC

S D

G

CGCS D

G

CGC

Cut-off Resistive Saturation

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

Dispositivi e modelli 39

Diffusion Capacitance

Bottom

Side wall

Side wallChannel

SourceND

Channel-stop implant NA1

SubstrateNA

W

xj

LS