Transcript BJTnHBT2015

Bipolar Junction Transistors and HBTs
EE 3311/7322 and EE 5312/7312
(draft)SMU
Bipolar Junction
Transistors and
Heterojunction
Bipolar Transistors
November 18, 2014
391 San Antonio, Palo Alto, CA
May, 2006—original buildings stored vegetables from farms
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~ 1958
~ 2006
Jaguar XK 140?
Playboy, August 1980 Issue (Bo Dereck)
PLAYBOY: What about your own
children? How did they turn out?
SHOCKLEY: …my children
represent a very significant
regression. My first wife… Two
of my three children graduated
from college—my daughter from
Radcliffe and my younger son from
Stanford. He … has obtained a
Ph.D. in physics. In some ways, I
think the choice of physics may be
unfortunate for him, because he has
a name that he will probably be
unlikely to live up to.
Playboy, August 1980 Issue (Bo Dereck)
PLAYBOY: What about your own
children? How did they turn out?
SHOCKLEY: …my children
represent a very significant
regression. My first wife… Two
of my three children graduated
from college—my daughter from
Radcliffe and my younger son from
Stanford. He … has obtained a
Ph.D. in physics. In some ways, I
think the choice of physics may be
unfortunate for him, because he has
a name that he will probably be
unlikely to live up to.
Excess Carriers and Quasi Fermi Levels
Recall:
Quasi Fermi Level Example
Consider a Si sample with
no = 1014 cm-3
nopo = ni2 so
po = 2.25x1020 cm-6/1014 cm-3
= 2.25x106 cm-3
By some means (optical
generation or electrical
injection), an additional 2x1013
cm-3 electron-hole pairs are
generated
Now,
n = 1.2x1014 cm-3
p = 2x1013 cm-3 (107 increase)
np = 2x1027 cm-3 , not ni2
Quasi Fermi Level Example, cont’d 1
Bipolar Junction Transistors
Bipolar means that both positive and negative charge carriers
contribute to current flow
FETs are unipolar—since only one type of charge carrier
contributes to current flow
Consider a pn junction:
Illuminated pn Junction
Bipolar Junction Transistor Concept
BJT Concept, cont’d 3
Properties of a p+n Junction
• If pp >> nn , then np is << pn
(since nopo = ni2 )
• Therefore, the current across
the depletion region is almost all
hole current
BJT Concept, cont’d 4
•
•
•
•
Emitter (forward biased p+n junction is the source of holes
Reverse biased np junction “collects” holes injected into the base
Holes are swept across the np depletion region by drift (
)
Need to make base narrow (< 1µm) to reduce recombination in base
BJT Concept, cont’d 5
Heavy doping also
decreases the bandgap
(not desirable), and
decreases resistance (desirable)
BJT Concept, cont’d 6
• IE is mostly hole current
• IC is mostly due to holes
injected into the base
• I E ~ IC
• IB is small, determined by
- electrons injected across the
p+n junction (5)
- recombination of injected
holes and electrons in the base
(1) and (4) [dominant term]
-electrons swept into the base
from the collector (3) (thermal
generation and electrons within
a diffusion length of the CB
junction edge)
BJT Concept, cont’d 7
BJT—no bias
BJT– with bias
BJT Concept, cont’d 8
BJT Concept, cont’d 9
Amplification in BJTs
Dominant component of base current is due to EHP
recombination in the base
Amplification in BJTs, cont’d 1
~ 100 is a typical number
Amplification in BJTs, cont’d 2
BJT as a Circuit Element
Why Heterojunction BJTs?
From J. Singh, Semiconductor Devices
Band Gap Shrinkage with Doping
Heterojunctions and Homojunctions
From J. Singh, Semiconductor Devices
Heterojunctions and Homojunctions, cont’d
Stop Here for 3311/7322
Heterojunction Conduction Band Offsets
AlGaAs/GaAs Heterojunction Conduction Band Offsets
InGaAs/GaAs Heterojunction Conduction Band Offsets
InGaAs/InAlAs/InP Heterojunction Conduction Band Offsets
Si/SiGe Heterojunction Conduction Band Offsets
Band Gap Shrinkage with Doping
Heterojunction Energy Band Diagram
Heterojunction Energy Band Diagram, cont’d 1
Heterojunction Energy Band Diagram, cont’d 2
Band Lineups: InP/InGaAs, InAlAs/InGaAs
Go to Schubert’s slides...
• Mention homework problems, too!
Emitter Injection Efficiency
Recall the IV equation
for a pn junction:
Consider an n+p
emitter base junction
In (4),
accounts for the finite length of the base
Emitter Injection Efficiency, cont’d 1
or
From Eqs. 3 and 4,
since
, Eq 5 becomes
(5)
Heterojunction Correction to Junction Current
For high doping, the bandgap of Si shrinks
Eq 7.11, Singh
Previously we determined,
so
+
+
• Emitter bandgap shrinks: exponential increase in base carriers injected into emitter
• Emitter bandgap increases: exponential decrease in base carriers emitted into emitter
Band Gap Shrinkage with Doping
For Silicon:
Heterojunction Junction Current, cont’d 1
So the emitter injection efficiency increases or decreases:
from
and using
we have
• Emitter bandgap shrinks: exponential decrease in performance
• Emitter bandgap increases: exponential increase in performance
Al0.3Ga0.7As/GaAs HBT
Eq. 7-81 of Streetman gives the
ratio of electron current to hole
current for a p-n heterojunction
Eq. 7-81 applies to a
homojunction BJT if the emitter is
heavily doped, which causes the
bandgap of Si to shrink
For a p-n heterojunction, the
bandgap difference between the
p and n region is 0.416 eV.
Such a bandgap difference gives
a factor of ~ 107 compared to a
homojunction BJT
Si Bandgap Dependence on Doping
http://ecee.colorado.edu/~bart/book/eband6.htm
Abrupt and Graded Interfaces
BJT and HBT Energy Band Diagram
BJT
•
•
•
•
HBT
ne and pb are majority carrier densities in the emitter and base
vn is the electron velocity in the base
vp is the hole velocity in the base
DEg is the bandgap difference
• Want high base doping (low resistance) for high speed—but lowers gain
• HBT wider bandgap solves problem by adding exp(DEg/(kT)) term to the gain
And Another Eg vs Lattice Constant Chart
EE7312
IntelliEPI HBT
Typical InGaAs HBT
Graded Base HBT
• Gradual change in bandgap induces an electric field due to the
bandgap gradient
• Adds a drift term in addition to diffusion current
Double Heterojunction HBT (DHBT)
Add Wide Band Gap Collector
• Eliminates the injection of holes into the collector
• Reduces saturation stored-charge density
• Speeds up device turnoff
Silicon-Germanium HBTs
• addition of Ge reduces the bandgap of Si
• lower effective electron mass in SiGe
AlGaAs/GaAs HBTs
InGaAs/InP HBTs
• Electron mobility in InGaAs is 1.6 times that of GaAs and
9 times higher than in Si
• highest ft values in InP based HBTs
Typical InGaAs/InP HBT
•
•
•
•
InGaAs has a smaller bandgap than Si or GaAs
requires lower voltage power supplies
better thermal properties than GaAs
surface recombination velocities smaller than GaAs
InGaAs/InP HBT Frequency Performance
Si/Si BJT
This slide is
unfinished—meant to
calculate the
degradation due to
highly doping the
collector with respect
to the base for a Si-Si
BJT and then on
another slide show
how to compensate
with a SeGe base HBT
Eq. 7-81 of Streetman gives the
ratio of electron current to hole
current for a p-n heterojunction
Eq. 7-81 applies to a
homojunction BJT if the emitter is
heavily doped, which causes the
bandgap of Si to shrink
For a p-n heterojunction, the
bandgap difference between the
p and n region is 0.416 eV.
Such a bandgap difference gives
a factor of ~ 107 compared to a
homojunction BJT
Ballistic Collection HBTs
ft ~ 105 GHz