strain-enhanced device and circuit for optical communication system

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Strain-enhanced Device and Circuit forStrain-enhanced Device and Circuit forOptical Communication SystemOptical Communication System

指導教授：劉致為博士學生：余名薪

台灣大學電子工程學研究所

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OutlineOutline Introduction

Optical Communication System Mechanical/Package Strain Technique

Strain-enhanced MOS Photodetector Strain-enhanced Transimpedance Amplifier 7Gb/s Transimpedance Amplifier SiGe HBT BiCMOS Active Inductor Summary and Future Work

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Optical Communication SystemOptical Communication System

MUX

CMU

Laser Driver

DFF

Q

TIA LA

CDR

DMUX

Optical Fiber

Data Input

Data Output

Photodetector & Transimpedance Amplifier (TIA)

System block diagram

Transmitter

Receiver

Medium

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Mechanical/Package Strain TechniqueMechanical/Package Strain Technique

Side view Top view

Mechanical setup (detector)

BiaxialTensile Strain

UniaxialTensile Strain

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O-ringto fix the substrate

Chip

Mechanical stress(at center)

Biaxial strain

r

Packagesubstrate

Package substrate Biaxial

compressive strain

Biaxial tensile strainChip

Mechanical displacement

O-ringChip

Mechanical stress(at centerline)

Uniaxial strain

Si strip

Si stripUniaxial

compressive strain

Uniaxial tensile strainChip

Mechanical displacement

Mechanical setup (circuit chip)

BiaxialTensile Strain

UniaxialTensile Strain

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Ramam & Electroluminescence spectra

Ramam spectrum EL spectrum

• The red-shift of Si-Si peak in Ramam spectra indicates 0.13% biaxial tensile strain and 0.35% uniaxial tensile strain.• The EL spectra of a MOS LED under tensile strain indicates bandgap shrinkage.


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Current change (%)At |VGS|-|VT|=1V

250nm node (L=240nm)

Compressive0.06% strain

Tensile0.06% strain

NFET

Uniaxial // channel

-4.3 10.8

Uniaxial ┴ channel

-6.9 2.5

Biaxial -6.2 8.7

PFET

Uniaxial // channel

5.5 -8.8

Uniaxial ┴ channel

-6.8 4.8

Biaxial 0.3 -0.9

Current enhancement in MOSFET

• The drain current is enhanced owing to the electron mobility enhancement.• NFET has enhancement under tensile strain, and PFET has enhancement under uniaxial compressive strain parallel to channel and the uniaxial tensile strain perpendicular to channel.

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OutlineOutline Introduction Strain-enhanced MOS Photodetector

Metal/Thin-oxide/P-Si Tunneling Diode Photodetector Responsivity Enhancement by Tensile Strain

Strain-enhanced Transimpedance Amplifier 7Gb/s Transimpedance Amplifier SiGe HBT BiCMOS Active Inductor Summary and Future Work

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MOS Tunneling Diode PhotodetectorMOS Tunneling Diode PhotodetectorI-V & C-V characteristics

I-V curve C-V curve

• At the negative bias region, the device can serve as a LED. When the gate is biased at the positive voltage greater than threshold voltage, the device can serve as a PD. • C-V indicates that the deep depletion in the NMOS tunneling diode is formed for the large positive gate bias.

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MOS Tunneling Diode PhotodetectorMOS Tunneling Diode Photodetector

Photo currentDark current

Interface defects

Δ EC

Δ EV

EC

EV

Al SiO2 P-Si

hv

EF

hv

Al SiO2 P-Si

EC

EV

Band diagram

Accumulation (LED) Inversion (Detector)

• The bandgap shrinkage enlarges the concentration of the electron in the bulk silicon, and since the tunneling process would not be the limiting factor for electron to go from p-Si to Al gate, the photo current increases. Namely, the responsivity is enhanced.

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MOS Tunneling Diode PhotodetectorMOS Tunneling Diode PhotodetectorMeasurement

Biaxial tensile strain Uniaxial tensile strain

• The dark current has almost no change under strain, and the photo current is enhanced gradually with increasing strain.

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MOS Tunneling Diode PhotodetectorMOS Tunneling Diode PhotodetectorResponsivity enhancement

12%

14%

• Uniaxial strain has potentiality to achieve higher strain gauge than biaxial strain.• The maximum of current enhancement is about 12% and 14% under biaxial and uniaxial strain respectively.

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OutlineOutline Introduction Strain-enhanced MOS Photodetector Strain-enhanced Transimpedance Amplifier

Circuit Design & Simulation Bandwidth Enhancement by Tensile Strain

7Gb/s Transimpedance Amplifier SiGe HBT BiCMOS Active Inductor Summary and Future Work

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Strain-enhanced Transimpedance AmplifierStrain-enhanced Transimpedance Amplifier

Iin

Vout

VDD

VB

50Ω

Parasitic isolation Core Amplifier Output buffer

M1

M2

M3 M4

M5 M6

M7

M8

M9

M10

R1

R2

R3

R4

R5

I1

I2

I3

X

Y Z

Circuit schematic

Main Gain Stage Peaking Stage

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Strain-enhanced Transimpedance AmplifierStrain-enhanced Transimpedance AmplifierInductive peaking

L

R

CVin

Vout

12

RCsLCs

RLsg

V

Vm

in

out

1

)1(22

sms

msRgm

• For m = 0.4, the transfer function exhibits a maximum flat response with a bandwidth improvement of 70% compared to a simple common source amplifier. If m = 0.7, the bandwidth reaches its maximum value with 1.5dB peaking and 85% improvement.

fM

agni

tude

(dB

) m = 0.4

m > 0.4

m < 0.4M1

M2

M3

I1

I2

Rf

VB3

Zin

(τ= RC , m = L/R2C )

Active Inductor

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f f

Main Gain Stage Overall Gain

Peaking Stage

Bandwidth enhancement

After strain After strain

8

7 8 7 8 7

1,gs

s sm m m m o

CL R

g g g g r

2262

2

2 nn

nnsm

Y

Z

ss

sRg

V

V

s

gsgds

gsgds

n L

CCR

CCL

106

1062

,)(

1

Active Inductor :

Peaking Stage :

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1E91E8 1E10

20

30

40

50

60

10

70

freq, Hz

dB(G

ain)

1E91E8 1E10

-150

-100

-50

0

50

100

150

-200

200

freq, Hz

Pha

se

Frequency response & Stability

Magnitude response Phase response

2 3 4 51 6

500

1000

1500

2000

0

2500

freq, GHz

Sta

bFac

t1

1 2 3 4 5 6 7 8 90 10

0.05

0.10

0.15

0.20

0.25

0.00

0.30

freq, GHz

mag(D

elta

)

K factor Δ factor

Gain: 60dBΩBandwidth: 3.5GHzPeaking < 0.5dB

K > 1

Δ< 1

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Strain-enhanced Transimpedance AmplifierStrain-enhanced Transimpedance AmplifierInput-referred noise current density & Eye diagram

Input-referred noise current density

Eye Diagram at 3.125Gb/s, 20uA

HzpAI innoise /202,

Jitter < 20ps

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Technology TSMC 0.18um RFCMOS

Supply voltage 1.8V

Transimpedance gain 60dBΩ

Bandwidth 3.5GHz

Input referred noise current

< 20pA/√Hz

Jitter < 20ps

Group delay variation < 65ps

Power consumption (core) 12mW

Chip Area 600*380 um2

Performance Summary

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Strain-enhanced Transimpedance AmplifierStrain-enhanced Transimpedance AmplifierMeasurement : Bandwidth enhancement

• Through 0.06% biaxial tensile strain, the characteristic of active inductor can be modified, thus improves the -3dB frequency.• The bandwidth enhancement is about 5.5%.

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Strain-enhanced Transimpedance AmplifierStrain-enhanced Transimpedance AmplifierCircuit layout & Die photograph

Die photo

Layout

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OutlineOutline Introduction Strain-enhanced MOS Photodetector Strain-enhanced Transimpedance Amplifier 7Gb/s Transimpedance Amplifier

Circuit Design & Simulation Measurement

SiGe HBT BiCMOS Active Inductor Summary and Future Work

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7Gb/s Transimpedance Amplifier7Gb/s Transimpedance Amplifier

R1

VB1

M1

M2

M3 M4

M5

R2

R3

L1

Rf

C2

Vout

C3

R6Iin

R4 R5

C1VB2

L2 L3

M6 M7

VB3

Parasitic isolation Core Amplifier Differential Output Buffer

Circuit schematic

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1E6 1E7 1E8 1E9 1E101E5 1E11

-50

0

50

-100

100

freq, Hz

dB(G

ain)

1E91E8 1E10

-150

-100

-50

0

50

100

150

-200

200

freq, Hz

Pha

se

7 8 96 10

50

100

150

200

0

250

freq, GHz

Sta

bFact1

10 20 300 40

0.2

0.4

0.6

0.8

0.0

1.0

freq, GHz

mag(D

elta

)

Frequency response & Stability


K factor Δ factor

Gain: 56dBΩBandwidth: 8GHzPeaking < 1dB

K > 1

Δ< 1

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7Gb/s Transimpedance Amplifier7Gb/s Transimpedance AmplifierInput-referred noise current density & Eye diagram

Input-referred noise current density

Eye Diagram at 10Gb/s, 20uA

HzpAI innoise /152,

Jitter < 15ps

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S11 S12

S21 S22

Measurement : S-parameters

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7Gb/s Transimpedance Amplifier7Gb/s Transimpedance AmplifierMeasurement : Frequency response


50)1)(1(

2

12212211

2121 SSSS

SZTransform function:

Gain: 57dBΩBandwidth: 6GHz

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7Gb/s Transimpedance Amplifier7Gb/s Transimpedance AmplifierMeasurement : Eye diagram

2.5 Gb/s PRBS 3.125 Gb/s PRBS

7 Gb/s PRBS

• Measured on PCB.• Equivalent input current ~ 50uA.• Eye can open well under 7Gb/s PRBS.• Jitter < 35ps @ 7Gb/s• The inductive characteristic of bond wire may help improve overall bandwidth.

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Simulation Measurement

Technology TSMC 0.18um RFCMOS TSMC 0.18um RFCMOS

Supply voltage 1.8V 1.8V

Transimpedance gain 56dBΩ 57dBΩ

Bandwidth 8GHz 6GHz

Input referred noisecurrent

< 15pA/√HzSensitivity:

20uA/eye open

Jitter < 15ps < 35ps @ 7Gb/s

Group delay variation < 80ps < 90ps

Power consumption (core) 10mW 10mW

Chip Area 880*980 um2 880*980 um2

Performance Summary

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7Gb/s Transimpedance Amplifier7Gb/s Transimpedance AmplifierDie & PCB photograph

Die photo

PCB photo

PCB layout

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OutlineOutline Introduction Strain-enhanced MOS Photodetector Strain-enhanced Transimpedance Amplifier 7Gb/s Transimpedance Amplifier SiGe HBT BiCMOS Active Inductor

CMOS Active Inductor BiCMOS Active Inductor

Summary and Future Work

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SiGe HBT BiCMOS Active InductorSiGe HBT BiCMOS Active Inductor

Zin

V1 V2

-Gm1

Gm2

I2

I1

C 21 mmin GG

sCZ

M1

M2 M1

M1

M1

M2

M2

M2

M1

M1

M1

M1

M2

M2 M2

M2

Basic configuration of active inductor

Gyrator-C topology:

CS-CD type

CS-CG type

Two-transistor active inductor:

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SiGe HBT BiCMOS Active InductorSiGe HBT BiCMOS Active InductorProposed active inductor

CMOSBiCMOS

Type A Type B

RsLs

Cp

Rp

Zin 2

1

mp gR

1gsp CC

21

1

mm

os gg

GR

21

2

mm

gss gg

CL

M1

M2

I2

I1

I2

M2

Q1

Q2

M1

I1

I2

I1

I2

M2

Q1

Q3 VB3

Equivalent model :

1

2

o

gs

s

s

G

C

R

LQ

2121

212 1TT

gsgs

mm

psSR ff

CC

gg

CLf

CascodeBiCMOS Type A

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)))(CC((

)CCC()(

211gd1gs22

gd1gd2gs21

gddsmm

dsin sCggsg

sgsZ

eq

m

gs

m

gd

mm

gsgd

gsm

gdm

mm

gs

mm

ds

in R

g

C

g

C

gg

CC

Cg

Cg

gg

C

gg

g

Z

2

2

2

1

22

2

21

222

21

22

1

2

2

2

21

1

1

11

]Re[

eq

m

gs

m

gd

mm

gsgd

gdmm

gs

mm

gs

in Lj

g

C

g

C

gg

CC

Cgg

C

gg

Cj

Z

2

2

2

1

22

2

21

222

221

22

21

2

1

1

]Im[

M1

M2

I2

Zin analysis

Zin

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1M 10M 100M 1G 10G 100G-100

-80

-60

-40

-20

0

20

40

60

80

100

Zin

ph

ase

Frequency (Hz)

CMOS BiCMOS BiCMOS cascode

Frequency response of input impedance

Magnitude Phase

• For fair comparison, all the active inductors are designed with identical MOSFET size and power consumption. • BiCMOS active inductor has wider inductive range.• The phase of BiCMOS type rises at lower frequency and reaches the higher degree, which means BiCMOS type has much higher quality factor and less resistive loss.

1M 10M 100M 1G 10G 100G0

10

20

30

40

50

60

70

Zin

mag

nit

ud

e (

dB

)

Frequency (Hz)


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0 2 4 6 8 10 12 14 16 18 20-20

-10

0

10

20

30

Ind

ucta

nce (

nH

)

Frequency (GHz)


0 2 4 6 8 10 12 14 16-5

0

5

10

15

20

Qu

ality

facto

r

Frequency (GHz)


Inductance & Quality factor

Inductance Q-factor

• BiCMOS type has much higher resonant frequency.• Cascode will reduce the resonant frequency owing to its extra parasitics, but it provides higher inductance at high frequency and higher quality factor.

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5 10 150 20

-1E-8

0

1E-8

2E-8

-2E-8

3E-8

freq, GHz

Indu

ctan

ce3

2 4 6 8 100 12

10

30

50

70

90

-10

110

freq, GHz

Q3

5 10 150 20

-1E-8

0

1E-8

2E-8

-2E-8

3E-8

freq, GHz

Indu

ctan

ce3

2 4 6 8 100 12

10

305070

90110

130150170

190

-10

210

freq, GHz

Q3

5 10 150 20

-2E-8

-1E-8

7E-24

1E-8

2E-8

3E-8

-3E-8

4E-8

freq, GHz

Indu

ctan

ce3

2 4 6 8 100 12

10

30

50

70

90

110

-10

130

freq, GHz

Q3

Inductor characteristic tuning

I1 tuning

I2 tuning

VB tuning

I1 increasingI1 increasing

I2 increasing I2 increasing

VB increasing VB increasing

Inductance Q-factor

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OutlineOutline Introduction Strain-enhanced MOS Photodetector Strain-enhanced Transimpedance Amplifier 7Gb/s Transimpedance Amplifier SiGe HBT BiCMOS Active Inductor Summary and Future Work

Summary CMOS Image Sensor

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SummarySummary A novel metal/thin-oxide/silicon structure tunneling diode photo detector is

proposed. With biaxial or uniaxial tensile strain, the band-gap of bulk Si shrinks, resulting in higher electron concentration under identical exposure, thus the responsivity is enhanced. The maximum of responsivity enhancement under biaxial and uniaxial tensile strain are about 12% and 14% respectively.

A Transimpedance Amplifier (TIA) adopting active inductor is designed. Through inductive characteristic tuning by biaxial tensile strain, a 5.5% bandwidth enhancement can be achieved.

A 7Gb/s transimpedance amplifier fabricated with TSMC 0.18um CMOS process is proposed. The measured gain and bandwidth are 57dBΩ and 6GHz respectively. The eye can open well with operation under 7Gb/s PRBS.

A novel BiCMOS type active inductor is proposed. From simulation, it can be proved that BiCMOS active inductor can achieve higher quality factor and resonant frequency than CMOS type with the great help from SiGe HBT. However, the inductance value would be slightly lower.

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Future WorkFuture WorkCMOS image sensor

Replace with MOS detector

• The photodiode can be replaced with our MOS tunneling diode photodetector by connecting the gate of the diode to the source of transfer gate.

• The lower dark current(~3nA/cm2) and higher quantum efficiency (~80%) can improve the performance of the pixel, such as dynamic range and sensitivity.

strain-enhanced device and circuit for optical communication system

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