Chapter
8
BIPOLAR JUNCTION
TRANSISTORS
Bipolar junction transistors are important in numerous technologies|ampliers, oscillators, high
speed logic. The following pages provide an overview.
BIPLOAR JUNCTION TRANSISTOR: STRUCTURE
Bipolar transistors
find important
uses as amplifiers,
drivers of other
devices, and
certain high speed
digital circuits.
Emitter contact
Base contact
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p+
p+ itaxyp
p
ne
n+
E +
n
n+
p
B
n ep
itax
y
n+
bur
ied
laye
r
p-ty
pe s
ubs
trat
e
Base width
Collector contact
p
n+
n+
C
+
n
p+
E
B
n+
Cross-sectional
view
© Prof. Jasprit Singh
A SCHEMATIC
CROSS-SECTION
axy
pit +
e
n
p
C
p
n
www.eecs.umich.edu/~singh
n+
HOW THE BASE CURRENT (BIAS) CONTROLS THE
EMITTER AND COLLECTOR CURRENT
EQUILIBRIUM: NO BIAS
Wb
–––– –
–––– –
––––––
~N
p=
ab
~N
n+ =
de
++ ++ +
––––––
~N
n=
dc
(a)
EMITTER-BASE JUNCTION
IS FORWARD BIASED
Electron current
IEn
–––– –
–––– –
––––––
––––
p
n+
+ ++ +
+
Collector current = BIEn
–
IEp
–
–
–
Hole current = Base current
– – –
n
(b)
Lowered emitter-base barrier allows injection of electrons from the
emitter into the base and the collector
Bipolar transistor operator on the basis of the base signal controlling the
potential barrier that electrons in the emitter see.
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
CURRENT COMPONENT IN A BIPOLAR TRANSISTOR
Base
contact
Emitter
contact
SiO2
n
p
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AAAA
Si
n
n'
A A'
Collector
contact
+
BE
Hole flow
IBE
p
IB
A
R
IBE
I
II
+
CB
R
IBE
VI
IBC
p
III
InBC V
Electron flow
InC
IE
IEB
n
n
IC
p
IV
n
A'
IEB
n = Emitter current injected into the base = IEn
IBE
p = Base current injected into the emitter = IEp
R
IBE = Recombination current in the base region
IBC
p = Hole current injected across reverse-biased base collector junction
IBC
n = Electron current injected across reverse-biased base collector junction
~ IC)
InC = Electron current coming from the emitter (=
What is needed for a high performance device?
• IEn >> IEp ; IBE ~ 0
high emitter efficiency
high base transport factor
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
MODES OF A BIPLOAR JUNCTION TRANSISTOR: MINORITY CHARGE DISTRIBUTION
VBE > 0
VCB < 0
–––– –
––––––
–––– –
––––––
VBE > 0
IE
VCB > 0
IC
–––– –
––––––
++ ++ +
+ + +
IB
VBE < 0
IE ~ 0
VCB > 0
IC ~ 0
+ ++
+ +
– – –
–––––
––––––
+ ++
+ +
IB ~ 0
Minority charge density
Minority charge density
Electrons
Electrons
nbo
Holes
nco
Minority charge density
Electrons
Holes
Holes
Holes
Holes
Holes
peo
SATURATION
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
FORWARD ACTIVE
– – –
CUTOFF
OPERATING CONFIGURATION OF A BIPOLAR JUNCTION TRANSISTOR
VCB
VEC
IE
C
C IC
E
IB
VBC
IB
E
IB
VEC
E
VEB
VBC
IE
Common base
Common emitter
(a)
IC
Common base
Common emitter
IE3
}
Cutoff
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IB2
IB1
IE2
© Prof. Jasprit Singh
Saturation
Active
IE1
IE = 0
VEC
VBC = 0
IE4
Active
~0.7 V 0
C
IC
Common collector
IC
Saturation
IE
B
B
B
VEB
VEB
IC
VBC
IB = 0
IB1
IB2
(b)
Cutoff
Reverse active
IB = 0
VEC
OPTIMIZATION OF A BIPOLAR TRANSISTOR
ISSUES: Emitter doping, base doping, collector doping, base width,
emitter thickness
EMITTER EFFICIENCY: γe ~ 1 –
peoDeWbn
nboDb
BASE TRANSPORT FACTOR: B ~ 1 –
2
Wbn
2L2b
Emitter doping >> base doping
Wbn << Lb
LOW OUTPUT CONDUCTANCE: Wbn should not change with collector bias
collector doping << base doping
REQUIREMENTS FOR A BIPOLAR DEVICE
• High gain
• High emitter efficiency
• High speed
DEMANDS AND PROBLEMS FOR A BIPOLAR JUNCTION TRANSISTOR
DEMANDS
PROBLEMS
Heavy emitter doping
Bandgap shrinkage causing base
injection
Low base doping
High base resistance
Narrow base width
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
EARLY VOLTAGE AND OUTPUT CONDUCTANCE OF I-V CURVES
Collector current α
1
Wbn
; Wbn = neutral base width
Base
Increasing VCE causes a
reduction in neutral base
width
collector current
increases.
VCE
Increasing
minority
carrier
gradient
xb = Wbn
xb = 0
(a)
If collector doping is much
smaller than base doping
the depletion width at base
collector junction will be
on the collector side
large early voltage, VA.
IC
VBE
|VA|
VCE
(b)
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
CURRENT GAIN DEPENDENCE ON BASE (COLLECTOR) CURRENT
Reduced β due to Kirk effect
and Auger effect
Log (IC, IB)
Base and collector
currents versus
forward bias voltage
In the range VEB <
~ 0.4
IC
IB
IB
exp
eVEB
mkBT
In the range VEB >
~ 0.4
IB
IB
exp
eVEB
kBT
Generation current
0.4
VEB (volt)
CURRENT GAIN, β
(a)
Recombination
current dominates
base current
Collector current reduced
due to base pushout
200
100
(b)
1 µA 10 µA 100 µA
1 mA 10 mA 100 mA
LOW INJECTION REGIME: Current gain is small because of non-ideal generationrecombination current.
HIGH INJECTION REGIME: Current gain drops because the effective base width is
pushed out into the collector + carriers (e-h) recombine due to Auger effect.
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
IC
SWITCHING DELAYS IN A BIPOLAR JUNCTION TRANSISTOR
VCC = 5 V
SWITCHING
CIRCUIT
RC
1 kΩ
OUTPUT
vo(t)
EXAMPLE PARAMETERS
VBE(on)
VBE(sat)
VCE(sat)
Cjeo
φe
me
RB
5 kΩ
INPUT
+
vi(t)
–
τF
= 0.7 V
= 0.8 V
= 0.1 V
= 0.5 pF
= 0.9 V
= 0.50
τBF
τS
Cjco
φc
mc
= 0.1 ns
= 10 ns
= 12 ns
= 0.2 pF
= 0.7 V
= 0.5
5V
INPUT VOLTAGE
vi
0
5.0 V
Vo
OUTPUT
t6 VOLTAGE
t5
t0
t1
t2
t4
0.1 V
t3
Minority
charge in the
base
Base
Base
Base
MINORITY CHARGE INJECTION
IMPORTANT ISSUE: Avoid going into deep saturation.
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
Base
Base
Base
MINORITY CHARGE REMOVAL
USE OF A SCHOTTKY JUNCTION FOR HIGH SPEED BIPOLAR DEVICES
MOTIVATION: Do not let the transistor go into deep saturation during switching.
Al
E
C
B
SiO2
n+
p
B
n
E
C
• Collector-Base reverse biased
Schottky diode is reverse
biased
Al makes an ohmic contact to
the p-type base and a Schottky
contact to the n-type collector
• Collector-Base forward biased
Schottky diode turns ON
and collector is bypassed
I
Schottky diode
Base-collector diode
Schottky diode is turned ON at a
voltage smaller than what it takes
the CBJ to be in the saturated mode
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
V
SMALL SIGNAL MODEL OF A BIPOLAR JUNCTION TRANSISTOR
C
B
EQUIVALENT CIRCUIT PARAMETERS
E
Base-Emitter Junction (forward biased)
re
= resistance between E and E'
Cπ
= diffusion capacitance
rπ
= junction resistance
Cje
= junction capacitance
rb
= resistance between B and B'
E
'
B
'
C
'
p-substrate
Collector-Emitter
gmVb'e' = current source
ro
= output resistance
Cs
= collector-substrate capacitance
(a)
Base-Collector Junction (reverse biased)
rµ
= junction resistance
Cµ
= junction capacitance
Cµ
rb
B
C
Cje rπ
gmVb'e'
E'
Ic
B
ro
Vbe
Cπ
E
re
(b)
Ib
Cs
rµ
Cπ
rc
C'
B
'
(c)
E
Cutoff frequency fT = 1
2πτec
τec = τe + τt + τd + τc
τe = EBJ capacitance charging time
τt = Base transit time =
Wb2
2Db
W
τd = Transit time through the collector depletion region = vdc
s
τc = Collector capacitance charging time
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
rπ
gmVbe
C
ADVANTAGES OF A HETEROJUNCTION BIPOLAR TRANSISTOR
REQUIREMENTS FOR A BIPOLAR DEVICE
• High gain
• High emitter efficiency
• High speed
DEMANDS AND PROBLEMS FOR A BIPOLAR JUNCTION TRANSISTOR
DEMANDS
PROBLEMS
Heavy emitter doping
Bandgap shrinkage causing base
injection
Low base doping
High base resistance
Narrow base width
SOLUTION: HETEROJUNCTION BIPOLAR TRANSISTORS
• Emitter can be heavily doped using a semiconductor with a bandgap larger than
the base semiconductor.
• Base can be heavily doped and be made narrow without increasing base resistance.
• Collector can be chosen from a material to increase breakdown voltage.
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh
HETEROJUNCTION BIPOLAR TRANSISTOR
Emitter
n GaAs
n GaAlAs
Large bandgap
emitter material
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p GaAs
Base
Small bandgap
base region
n GaAs
n+ GaAs
Small bandgap
collector region
n+ GaAs
Emitter
(n)
Emitter
(n)
Base
(p)
Barrier for
electron
injection
– – – –
Barrier for
electron
injection
– – – –
Base
(p)
Egb
+ + + +
Ege
++ ++
Barrier for
hole injection
++
Homojunction transistor
Current gain (HBT) = current gain (BJT) x exp
Heterojunction transistor
∆Eg
kBT
( (
• HBT concept allows high base doping and still maintain high emitter efficiency.
This allows one to make very thin base devices with low base resistance.
© Prof. Jasprit Singh
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Barrier for
hole injection
AN OVERVIEW OF ADVANCES IN BIPOLAR TECHNOLOGY
Silicon bipolar technology
• Advanced fabrication techniques
are allowing devices with fT ~25
GHz
Advanced fabrication techniques
• Self-aligned emitter base
• Trench isolation to avoid cross-talk (SiO2 fills
the "trenches").
• Sidewall contacts. Polysilicon is used to contact
the base.
• Polysilicon emitter contact provides low
recombination at the contact and suppresses base
injection into the emitter.
Si-based HBTs
• Si/SiGe HBTs have shown
remarkable promise. Cutoff
frequencies approaching 100 GHz
have been demonstrated.
Si can be combined with
• amorphous silicon (Eg = 1.5 eV)
• β-SiC
(Eg = 2.2 eV)
• polysilicon
(Eg = 1.5 eV)
Most promising combination is Si/SiGe, which
can be fabricated by epitaxial growth.
GaAs/AlGaAs HBTs
• fT of ~100 GHz has been
demonstrated.
• Excellent quality of interface allows fabrication
of high-quality HBTs.
• Devices can be monolithically integrated with
optoelectronic devices.
InGaAs/InAlAs and
InGaAs/InP HBTs
• fT of ~175 GHz has been
achieved.
• In0.53Ga0.47As is lattice-matched to InP and
In0.52Al0.48As.
• High-quality HBTs can be produced and
integrated with optical devices.
© Prof. Jasprit Singh
www.eecs.umich.edu/~singh