ece-305: spring 2018 bjt and hbt nonidealities
TRANSCRIPT
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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
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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)
Bermel ECE 305 S18
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
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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
Bermel ECE 305 S18
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
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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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Bermel ECE 305 S18
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
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doping for maximum gain
n+emitter
pbase
ncollector
n+
FB RB
IEn
IEp
Bermel ECE 305 S18
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
Bermel ECE 305 S18
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
Bermel ECE 305 S184/23/2018
AE
AC
AC >> AE
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Low Base Width punch-through
Bermel ECE 305 S18
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
Bermel ECE 305 S18
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
Bermel ECE 305 S18
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) …
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how to make better transistors
Bermel ECE 305 S18
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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Bermel ECE 305 S18
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
Bermel ECE 305 S18
EG Si 1.12 eV
EG Ge 0.66 eV
EG Si 1.12 eV
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Bermel ECE 305 S18
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
Bermel ECE 305 S184/23/2018
https://www.macrumors.com/2017/11/02/fight-for-space-iphone-x/
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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
Bermel ECE 305 S184/23/2018
Choose an emitter with a higher bandgap than the base
IEn µniB2
NAB
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SiGe HBTs
Si SiGe Si n+
FB RB
Bermel ECE 305 S18
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
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poly-silicon HBT emitter
Bermel ECE 305 S18
N+
P N
SiGe intrinsic base Dielectric trench
N+P+
N
P-
N-N+
CollectorEmitterBase
N+
Poly-silicon
emitter
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Bermel ECE 305 S18
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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Bermel ECE 305 S18
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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Bermel ECE 305 S18
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
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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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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
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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?
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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
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Emergence of Macroelectronics
Areasmall medium large
Perf
orm
ance
low
m
ed
ium
h
igh
mesoporous
NanoNetPoly-Si
Polymers
Flexible ElectronicsEnergy Biosensors
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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
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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)
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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
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Superhydrophobic coatings
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https://www.youtube.com/watch?v=16RcYoXOvRM
Anti-soiling coatings for photovoltaic modules
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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
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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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