Lecture 6. CMOS Device (cont)

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© Digital Integrated Circuits2nd Devices

Lecture 6. CMOS Device (cont)Lecture 6. CMOS Device (cont)

ECE 407/507

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NoticeNotice

Reading Assignment : chapter 1, chapter 3 (finish reading)

Both hw1 and lab1 are on the website hw1 due in one week (next Thurs.) Lab1 due in two week (the Thurs. after

next )

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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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0.5 1 1.5 2 2.50

1

2

3

4

5

6

7x 10

5

VDD

(V)

Req

(O

hm)

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C GCB_1 C GCS C GCD

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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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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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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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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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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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Parasitic ResistancesParasitic Resistances

W

LD

Drain

Draincontact

Polysilicon gate

DS

G

RS RD

VGS,eff

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Future PerspectivesFuture Perspectives

25 nm FINFET MOS transistor

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New Tech: Silicon On Insulator (SOI)New Tech: Silicon On Insulator (SOI)

Silicon wafers are highly perfect : critically important for achieving high device yield.

But a more radical change may be needed in the material structure, processing method, or device design in order to enhance the circuit performance.

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Why use SOIWhy use SOI

Extend the life of traditional silicon technology

Boost speed Reduce power consumption Solve some scaling difficulties

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Transistor crosssectionTransistor crosssection

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SOI material structure SOI material structure

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Benefits of SOI -performanceBenefits of SOI -performance

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Benefits of SOI -- powerBenefits of SOI -- power

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Benefits of SOI – timing Benefits of SOI – timing

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SiGe: Silicon Germanium SiGe: Silicon Germanium

Used to be inefficient in chip production Extremely high frequencies: 60Ghz Very little power usage 70% faster, 35% less power

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Why SiGeWhy SiGe

The layer of latticed silicon and germanium added to the chips silicon layer increases the distance between

silicon atoms

Less force between atoms, easy for electrons to pass by with less resistance

IBM suggests combining SiGe and SOIIBM suggests combining SiGe and SOI

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Thermal problem with SiGeThermal problem with SiGe

The diagram above shows the effect of localized self-heating in the emitters(30C for 40mv)

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Lecture 6. CMOS Device (cont)