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 USApbermel@purdue.edu

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

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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

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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

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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)

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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

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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

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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+

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“inverted” base doping

Wide gapemitter

pbase

ncollector

n+

FB RB

IEn

IEp

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log10 ND NA { }NDE

NAB

NDC

ND

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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

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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

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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

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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.

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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

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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

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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

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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)

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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.

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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

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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

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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

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Major Questions for Semiconductor Researchers

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• 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?

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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.

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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

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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).

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Artificial Skin

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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

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