ece-305: spring 2018 bjt and hbt nonidealities

ECE-305: Spring 2018

BJT and HBT Nonidealities

Professor Peter BermelElectrical and Computer Engineering

Purdue University, West Lafayette, IN [email protected]

Pierret, Semiconductor Device Fundamentals (SDF)Chapters 10 and 11 (pp. 371-385, 389-403)

4/23/2018 Bermel ECE 305 S18

MOSFETs vs. BJTs

IC

IBVCE

IE

VBEsiliconS

D

G

VBE

VDS

IG 0

ID

MOSFET characteristics:• simple to make• no gate current• moderate gm• low capacitance

BJT characteristics:• more complex to make• base current• large gm• high capacitance

C

E

B

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active region example 1 (NPN)

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AE 10 m ´ m

IC 50 A

NDE 1.5 ´1019 cm3

NAB 1.0 ´1017 cm3

NDC 2.0 ´1016 cm3

WB 0.25 m

WB 0.50 m

WC 1.50 m

t pE 0.1 ns

t nB 75 ns

t pC 150 ns

4/23/2018 8

Ebers-Moll parameters summary

IC VBE ,VBC aF IF0 eqVBE kBT 1 IR0 eqVBC kBT 1

IE VBE ,VBC IF0 eqVBE kBT 1 a RIR0 eqVBC kBT 1

a R aFIF0IR0

aF 0.9987

IR0 1.91´1016A

IF0 1.33´1016A

a R 0.70 bR a R1a R

2.3

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What is IC?

VCE 0

IE

IC

IB

IB 50 nA

IC aF IF0 eqVBE kBT 1 IR0 eqVBC kBT 1

IE IF0 eqVBE kBT 1 aRIR0 eqVBC kBT 1

VCE VBE VBC 0

VBC

VBE

VBC VBE

IC aF IF0 IR0 eqVBE kBT 1

IE IF0 aRIR0 eqVBE kBT 1 4/23/2018 Bermel ECE 305 S18 10

What is IC?

VCE 0

IE

IC

IB

IB 50 nA

IB IE IC

IB 1aF IF0 1aF IR0éë ùû eqVBE kBT 1

VBE

VBE

IC aF IF0 IR0 eqVBE kBT 1

IE IF0 aRIR0 eqVBE kBT 1

eqVBE kBT 1 IB1aF IF0 1aF IR0éë ùû

Bermel ECE 305 S184/23/2018 11

What is IC?

IC aF IF0 IR0

1aF IF0 1aF IR0

ìíîï

üýþïIB

IB 50 nA

aF 0.9987

IR0 1.91´1016A

IF0 1.33´1016A

a R 0.70

IC 1.01´ IB

IB 50 nA

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result

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VCE

E: emitter

C

B: base

IC

NPN BJT

IC

IE

IB 50 nA bF 768

IB 50 nA

IC 0.38 mA

IC IB

IC 0 atVCE VOS

Why?4/23/2018 13

Gummel Plot and Output Characteristics

2 2, , //( 1) ( 1)BCBEi B i B q kTq kTn n

B B B B

C VVn nqD qDe e

A W N W N

I ;

2, /( 1)BEp i E V kT

E E

B qqD n

eA W N

I

CDC

BI

Ib

DCbCommon emitter Current Gain

VBE

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=

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NE

NB

NC

Emitter doping: As high as possible withoutband gap narrowing

Base doping: As low as possible, withoutcurrent crowding, Early effect

Collector doping: Lower than base dopingwithout Kirk Effect

Base Width: As thin as possible withoutpunch through (~1 mm in ‘50s, 200 Å now)

4/23/2018

2,

2,

i Bn E EDC

B p i E B

nD W N

W D n Nb

How to make a Good Silicon Transistor

~1, same material

15

doping for maximum gain

n+emitter

pbase

ncollector

n+

FB RB

IEn

IEp

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log10 ND NA { }

NDE NAB

NDC

ND

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doping for maximum gain

n-collector

n+

n+

IBIB

high base doping lowers the base resistance

high sub-collector doping lowers collector series resistance

light collector doping increases the breakdown voltage

4/23/2018 Bermel ECE 305 S18 17

emitter-base doping

n+emitter

pbase

ncollector

n+

FB RB

IEn

IEp

IEn µni2

NAB

IEp µni2

NDE

High emitter doping increases the emitter injection efficiency.

g F 1

1DpE

DnB

WB

WE

NABNDE

NDE >> NAB

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High emitter doping benefits

n+emitter

pbase

ncollector

n+

FB RB

IEn

IEp

IEn µni2

NAB

IEp µni2

NDE

High emitter doping increases the emitter injection efficiency.

g F 1

1DpE

DnB

WB

WE

NABNDE

NDE >> NAB

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high emitter doping bandgap narrowing

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2,

2,

i Bn E

B p i E B

EnD W

W D n

N

Nb

Band-gap narrowing reduces gain significantly …

(Esaki-like) tunneling cause loss of base control …

,

,

/

/

B

g E

gE kT

n C VE

B p BC V

EE

kT

D N NW e

W D N e NN

N

/gE kT E

B

Ne

N

D

4/23/2018 20

Low base doping emitter current crowding

n-collector

n+

n+

IBIB

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AE

AC

AC >> AE

21

Low Base Width punch-through

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NE

NB

NC

0

,

2 s Ep BE bi BE

B E B

k Nx V V

q N N N

0,

2 s Cp BC bi BC

B C B

k Nx V V

q N N N

NN+

4/23/2018 22

“inverted” base doping

Wide gapemitter

pbase

ncollector

n+

FB RB

IEn

IEp

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log10 ND NA { }NDE

NAB

NDC

ND

4/23/2018 23

High collector doping Kirk effect

n+emitter

pbase

ncoll

n+

FB RB

IEn

IEp

r qND

ICn qAEDnWB

Dn 0

IC qAWnusat n ND

“base push out”/ Kirk effect impliescannot reduce base doping arbitrarily

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Low base doping early effect

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2,

2. . ,

i Bn E EDC

p B p C i EB p B

D W n N

W x x D n Nb

2 2, ,( / ) ( / )

, ( 1) 1' '

( )BE BCi B i BqV kT qV kTn nn

B

C

B B B

qD n qD nI e e

NW W N

C C C

BC BC A A

dI I I

dV V V V

VBC

VA

IC

Ideally

In practice

Ref. Fig 11.5 c,d B B

A

CB

qN WV

C

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Gain depends on collector voltage (bad) …

25

how to make better transistors

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2,

2,

i Bn E E

B p i E B

nD W N

W D n Nb

Classical Shockley Transistor

Heterojunction Bipolar Transistor

Graded Base transport

Polysilicon Emitter

4/23/2018 26

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heterojunction bipolar transistors (HBTs)

A heterojunction bipolar transistor

Shockley realized that HBT is possible, but Kroemer really provided the foundation of the field and worked out the details.

Kroemer

4/23/201827

SiGe HBTs

Si SiGe Si n+

FB RB

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EG Si 1.12 eV

EG Ge 0.66 eV

EG Si 1.12 eV

4/23/2018 28

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SiGe HBT applications

1) optical fiber communications-40Gb/s…….160Gb/s

2) Wideband, high-resolution DA/AD convertersand digital frequency synthesizers

-military radar and communications

3) Monolithic, millimeter-wave IC’s (MMIC’s)

-front ends for receivers and transmitters

future need for transistors with 1 THz power-gain cutoff freq.

4/23/2018 29

SiGe HBTs

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https://www.macrumors.com/2017/11/02/fight-for-space-iphone-x/

30

HBT (wide gap emitter)

n+emitter

pbase

ncollector

n+

FB RB

EGE EGB EGC

IEn

IEp

IEn µniB2

NAB

IEp µniE2

NDE

g F 1

1DpE

DnB

WB

WE

NABNDE

niE2

niB2

ni2 µ eEG kBT

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Choose an emitter with a higher bandgap than the base

IEn µniB2

NAB

31

SiGe HBTs

Si SiGe Si n+

FB RB

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EG Si 1.12 eV

EG Ge 0.66 eV

EG Si 1.12 eV

4/23/2018

g F 1

1DpE

DnB

WB

WE

NABNDE

niE2

niB2

ni2 µ eEG kBT

emitter collectorbase

Choose an emitter with a higher bandgap than the base

32

poly-silicon HBT emitter

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N+

P N

SiGe intrinsic base Dielectric trench

N+P+

N

P-

N-N+

CollectorEmitterBase

N+

Poly-silicon

emitter

4/23/2018 33

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mesa HBTs

nemitter

ncollector n+

EG1>EG2

p+base

EG2 EG3>EG2

p+ basen-collector

n+

semi-insulating substrate

nmesa HBT

4/23/2018 34

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collector breakdown

VBE

Common Emitter(IE variable, IB fixed)

Common emitter breakdown voltage is smaller than common base breakdown voltage.

Common Base(IE fixed, IB variable)

4/23/201835

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conclusions

• There are many non-ideal effects of bipolar

junction transistors that give rise to design

constraints

• Classical (MSM or Shockley) BJTs are

almost nonexistent today.

• Modern BJTs use poly-Silicon emitter, graded

based, hetero E-B junction, etc., to enhance

performance.

4/23/2018 36

Summary of ECE 305

Semiconductor Devices Fundamentals

Drift-diffusion model for semiconductors

D AD q p n N N

1

JP P P

pr g

t q

P P Pqp qD pJ E

1N N N

nr g

t q

J

N N Nqn qD nJ E

Band-diagram

Diffusion approximation,Minority carrier transport,

Ambipolar transport

4/23/2018 Bermel ECE 305 S18 38

Short-cut to Band-diagram

DN AN

… is equivalent to solving the Poisson equation

Vacuum level

EC

EV

EF

c2c1

Neutral Neutral

Space Charge

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39

Minority Carrier Diffusion

( )

( )

( 0 )

( 0 )

b

b

n i

p i

F Ei

F E

i

n x n e

p x n e

( )2 2b b pn AqV

i i

F Fn e n enp

Fp

Fn

q(Vbi-VA)

-VA

2

(0 ) b Aqi

A

Vn eN

n

(0 ) Ap N

AN

0

2

(0 ) (0 ) (0 )

1b

D

G G

A

V V

qVi

A

n n n

ne

N

4/23/2018 Bermel ECE 305 S1840

Small Signal AC Response

RS

G

CD

CJ

x

Δρ

x

Δρ

VA<0

VA>0

np

x4/23/2018 Bermel ECE 305 S18

41

Concepts for Device Analysis

Equilibrium DC Small signal

Large Signal

Diode Band-diagram diffusion dn/dt~jwn Charge-control

Schottky Band-diagram TE Junction

capacitance

Majority transport

BJT/HBT Band-diagram diffusion/TE dn/dt~jwn Charge-control

MOSCAP

MOSFET

2D band-diagram

Drift/TE MOS capacitance

Charge-control

4/23/2018 Bermel ECE 305 S1842

Major Questions for Semiconductor Researchers

4/23/2018 Bermel ECE 305 S18

• Can we achieve computing power equivalent

to or greater than that of the human brain?

• Can we create and deploy sustainable energy

technologies to transition away from fossil

fuel usage?

• Can we integrate electronics with biology to

detect and fix debilitating diseases?

43

Grand Challenges in Electronics

1906-1950s 1947-1980s 1980-until now

Vacuum Tubes

Bipolar MOSFET

Spintronics

Bio Sensors

Displays ….

Now ??

?

1900 1920 1940 1960 1980 2000 2020

Te

mp

Tubes Bipolar MOS

4/23/2018 Bermel ECE 305 S1844

Emergence of Macroelectronics

Areasmall medium large

Perf

orm

ance

low

m

ed

ium

h

igh

mesoporous

NanoNetPoly-Si

Polymers

Flexible ElectronicsEnergy Biosensors

45

Thin Film Organic Transistors

www.faculty.iu-bremen.de/dknipp/group/research.htmM.G. Kanatzidis, Nature, 428, 2004.

pentacene

Samsung flexible phone

Can you draw the band-diagram?What type of transport theory would you use? Would you be able to use numerical simulators

from nanohub.org to explore the TFT?


PV Simulations on nanoHUB.orga major resource for computational nanotechnology

enabled by the HUBzero platform for simulation, learning, and collaboration

• 25 Hubs operating or under construction• open source platform• 6,144 dedicated cores with over 17,000 on standby

4/23/2018 Bermel ECE 305 S18 47

Multijunction Photovoltaics

“ The NREL researchers improved the cell’s efficiency by enhancing the photon recycling in the lower, gallium-arsenide junction by using a gold back contact to reflect photons back into the cell, and by allowing a significant fraction of the luminescence from the upper, GaInP junction to couple into the GaAs junction. Both the open-circuit voltage and the short-circuit current were increased.”

—NREL News Release, June 24, 2013

• The new record obtain in June 2013, at 31.1%, is a significant jump from 29.5% prior record• Enhanced photon recycling is believed to cause this improvement.

4/23/2018 Bermel ECE 305 S18 48

Technology: Sequencing by Synthesis

Sanger method (1990s) Babbage computer (~1830s)

Intel Chip TodayIon-torrent system (2011)

4/23/2018 Bermel ECE 305 S18 49

Technology: Sequencing by Synthesis

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Superhydrophobic coatings

4/23/2018 Bermel ECE 305 S18

http://wyss.harvard.edu/viewpage/316

SLIPS Coatings ‘Waterproof’ circuit boards

http://www.cytonix.com/conformal-coating-s/1872.htm

51

Superhydrophobic coatings

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https://www.youtube.com/watch?v=16RcYoXOvRM

Anti-soiling coatings for photovoltaic modules

52

Artificial Skin

4/23/2018

M.L. Hammock et al., "25th Anniversary Article: The Evolution of Electronic Skin (E‐Skin): A Brief History, Design Considerations, and Recent Progress.“ Advanced Materials 25, 5997-6038 (2013).


Artificial Skin

4/23/2018 Bermel ECE 305 S18

Multiplex, flexible strain-gauge sensor based on the reversible interlocking of Pt-coated polymer nanofibres. Image: Nature Materials (2012) http://dx.doi.org/10.1038/nmat3380

54

Developing New Ideas

“New ideas pass through three periods: 1) It can't be done.2) It probably can be done, but it's not worth

doing.3) I knew it was a good idea all along!”

“I don't pretend we have all the answers. But the questions are certainly worth thinking about.”

-- Sir Arthur C. Clarke

4/23/2018 Bermel ECE 305 S1855

ece-305: spring 2018 bjt and hbt nonidealities

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