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ECE 255:
Bipolar Junction Transistors (BJTs)
(Sedra and Smith, 7th Ed., Sec. 6.1)
Mark Lundstrom School of ECE
Purdue University West Lafayette, IN USA
ECE 255: Fall 2019 Purdue University
BJT’s
Lundstrom: Fall 2019 2
1) Transistors 2) E-band review of PN junctions 3) E-band treatment of BJT’s 4) Active region IV 5) Saturation region IV 6) Cutoff region IV
“BJT” = Bipolar Junction Transistor
The transistor as a “black box”
control
terminal 1
terminal 2
I1
black box
Lundstrom: Fall 2019
terminal 4
3
A small current (or voltage) on the control terminal controls a much larger current through two other terminals.
IV characteristics: Resistor
R
I
V
I I = V R
more resistance
less resistance
V
+
−
Ohm’s Law
Georg Ohm, 1827
I = V R
Lundstrom: Fall 2019 4
IV characteristics: Ideal current source
VI0
V
II = I0+
−
I
I = I0
Lundstrom: Fall 2019 5
IV characteristics: Transistors
VCE
IC
VBE1, IB1
“resistor” “saturation
region”
base controlled current source “active region”
6
E: emitter
B: base
IC
NPN BJT
IE
IB
C: collector “output characteristics”
only leakage currents “cut-off region”
IV characteristics: Real current sources
V
II0
VI0
+
−
R0
I
I = I0 +V R0
Lundstrom: Fall 2019 7
IV characteristics: Real transistors
VCE
IC
VBE1, IB1
Lundstrom: Fall 2019 8
output resistance
E: emitter
B: base
IC
NPN BJT
IE
IB
C: collector
“output characteristics”
Applications of BJT’s symbol
Lundstrom: Fall 2019 9
E: emitter
B: base
IC
NPN BJT
IE
IB
C: collector
C
E B
switch amplifier
input signal
output signal
E
C
B
SiGe HBTs
Martin Claus TU-Dresden Lundstrom: Fall 2019
10 Circuit board of an iPhone 5
n-collector
BJT structures: Double diffused BJT
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p base n+
C B B E
Lundstrom: Fall 2019
Silicon wafer
n+
NPN BJT operation: Active region
n+ emitter
p base
n collector
n+
FB RB
To understand this device, we just need to understand PN junctions.
C
B
B
E C N
P
N
circuit symbol
Lundstrom: Fall 2019 12
PNP BJT operation: Active region
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p+ emitter
n base
p collector
p+
FB RB
C
B
B
E C
Lundstrom: Fall 2019
This is not a BJT
Lundstrom: Fall 2019
C
B
E
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BJT’s
Lundstrom: Fall 2019 15
1) Transistors 2) E-band review of PN junctions 3) E-band treatment of BJT’s 4) Active region IV 5) Saturation region IV 6) Cutoff region IV
“BJT” = Bipolar Junction Transistor
NP Junction in equilibrium
Lundstrom: Fall 2019 16
ΔE = qVbi
E
x
EC x( )
EV x( )
N-type P-type
Vbi =kBTqln NAND
ni2
⎛⎝⎜
⎞⎠⎟
nn0 = ND
pp0 = NA
np0 = ni2 NA
Forward biased junction
Lundstrom: Fall 2019 17
q Vbi −VA( ) EC x( )
EV x( )
IN→P
IP→N
electron energy A FB junction injects (emits) electrons from the N-side across the junction and into the P-side.
A FB junction also injects holes from the P-side across the junction and into the N-side.
N-type P-type
Forward biased NP junctions
Lundstrom: Fall 2019
q Vbi −VD( )
InIn = ISne
qVD kBT
WP
WN
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I pISp ∝ pn0 =
ni2
ND
ISn ∝ np0 =ni2
NA
I p = ISpeqVD kBT
ISnISp~ ND
NA“electron injection efficiency”
ND NA
Reverse biased junction
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q Vbi −VA( )
EC x( )
EV x( )
IN→PVR = −VA A RB junction collects minority carrier electrons from the P-side.
A RB junction collects minority carrier holes from the N-side.
N-type P-type
BJT’s
Lundstrom: Fall 2019 20
1) Transistors 2) E-band review of PN junctions 3) E-band treatment of BJT’s 4) Active region IV 5) Saturation region IV 6) Cutoff region IV
“BJT” = Bipolar Junction Transistor
Energy band treatment of a BJT
https://www.pbs.org/wgbh/americanexperience/features/silicon-timeline-silicon/ 21
Interesting video on the invention of the transistor:
BJT: Equilibrium E-band diagram
Lundstrom: Fall 2019 22
E
x
EC x( )
EV x( )
N-type emitter
P-type base
N-type collector
qVbi1 qVbi2
NDE
NDC
NAB
BJT: Active region E-band diagram
Lundstrom: Fall 2019 23 x
EC x( )
EV x( )
N-type emitter
P-type base N-type
collector FB emitter-base junction injects electrons in the base
RB collector-base junction collects electrons that diffuse across the base
BJT: Active region E-band diagram
Lundstrom: Fall 2019 24 x
EC x( )
EV x( )
N-type emitter
P-type base
N-type collector
IE ≈ IC = ISeqVBE kBT
almost independent of collector voltage
NDE >> NAB
BJT’s
Lundstrom: Fall 2019 25
1) Transistors 2) E-band review of PN junctions 3) E-band treatment of BJT’s 4) Active region IV 5) Saturation region IV 6) Cutoff region IV
“BJT” = Bipolar Junction Transistor
NPN BJT operation (general)
n+ emitter
p base
n collector
n+ IE IC
IB
IEn ICn
IEp ICp
FB/RB FB/RB
Lundstrom: Fall 2019
In general, four currents, two for each junction
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Also four regions of operation
BJT operation: Active region
n+ emitter
p base
n collector
n+
FB RB
x
Just two PN junctions (that interact).
Lundstrom: Fall 2019 27
NPN BJT operation (active)
n+ emitter
p base
n collector
n+ IE IC
IB
IEn ICn ≈ IEn
IEp
IEn >> IEp
FB RB
IC ≈ IEn
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IC = ISeqVBE kBT
IC ≈ IE
BJT in active region
VCE
E: emitter
C: collector
B: base
IC
NPN BJT
IC
VBE1, IB1
IE
IB
29 Lundstrom: Fall 2019
(forward) active region EB: FB, BC: RB
IC = ISeqVBE kBT
Early effect: IC = ISeqVBE kBT 1+VCE VA( )
Base current
n+ emitter
p base
n collector
n+ IE IC
IB
IEn ICn ≈ IEn
IEp
IEn >> IEp
FB RB
Lundstrom: Fall 2019
IEp ≈ IB
IEn ≈ IC
10 < β <1000
IB =ISβeqVBE kBT << IC
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ICIB
≡ βIC >> IB
BJT in active region (beta = 100)
VCE
E: emitter
C: collector
B: base
IC = 1.0 mA
NPN BJT
IC
IE(forward) active region
EB: FB, BC: RB
31 Lundstrom: Fall 2019
1 mA
IB = 0.01 mA
IB = 0.01 mA
BJTs at low VCE
VCE
E: emitter
C: collector
B: base
IC
NPN BJT
IC
IB
IE
IB
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IC
What happens here (at low VCE)? Lundstrom: Fall 2019
NPN BJT at low VCE
VCE
VBE
VCB
VBE +VCB = VCE
KVL:
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Active region:
VBE ≈ 0.7 VIf:
VCE <VBE
VCB < 0
0.0 V
0.7 V
0.2 V
The base-collector junction is forward biased! Lundstrom: Fall 2019
N
N
P
NPN BJT operation (saturation)
n+ emitter
p base
n collector
n+
FB FB
IE IC
IB
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IEn ICn
FB FB Less collector current and more base current.
IC ≠ β IB
Three regions
Lundstrom: Fall 2019
VCE
E: emitter
C: collector
B: base
IC
NPN BJT
IC
VBE1, IB1
IE
IB
(forward) active region EB: FB, BC: RB
saturation region EB: FB, BC: FB
cut-off region EB: RB, BC: RB
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IC = β IB
What is the fourth region?
Lundstrom: Fall 2019
E: emitter
C: collector
B: base
IC
NPN BJT
IC
IE
IB
1) (forward) active region EB: FB, BC: RB
2) saturation region EB: FB, BC: FB
3) cut-off region EB: RB, BC: RB
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4) (reverse) active region EB: RB, BC: FB
Summary
37 Lundstrom: Fall 2019
A BJT consists of two, interacting PN junctions.
In the forward, active region, the EB junction is forward biased and the BC junction is reverse biased.
In the active region, a small base current produces a much larger emitter current.
BJTs come in two flavors – NPN and PNP.
In the saturation region, the collector current decreases and the base current increase.
Summary
38 Lundstrom: Fall 2019
Emitter-Base Base-Collector
Active region:
Saturation region:
Cut-off region:
FB RB
FB FB
RB RB
Equations to know
39 Lundstrom: Fall 2019
IC = ISeVBE VT
IC = ISeVBE VT 1+VCE VA( )
VT = kBT q = 0.026 (T = 300 K)
BJT’s
Lundstrom: Fall 2019 40
1) Transistors 2) E-band review of PN junctions 3) E-band treatment of BJT’s 4) Active region IV 5) Saturation region IV 6) Cutoff region IV
