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CHAPTER 5: BIPOLAR
TRANSISTORS &
RELATED DEVICES
INTRODUCTION
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The bipolar junction transistor (BJT) is a
semiconductor device constructed with three doped
regions
The most common use of the BJT is in linear
amplifier circuits (linear means that the output is
proportional to input). It can also be used as a switch
(in, for example, logic circuits).
What’s meaning by BJT?
What’s the difference between PNP & NPN transistor?
Transistor Mode Operation and Characteristic I-V
curves
 The current gain
 The cutoff frequency and switching time of a
bipolar transistor
 The advantages of heterojunction bipolar
transistor
 The power handling capability of thyristor and
related bipolar devices
The Bipolar Junction Transistor
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The term Bipolar is because two type
of charges (electrons and holes) are
involved in the flow of electricity
The term Junction is because there
are two p-n junctions
There are two configurations for this
device
NPN and PNP Transistors
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NPN is more widely used
Majority carriers are electrons so it
operates more quickly
PNP is used for special applications
The terminals of the transistor are
labeled (Base, Emitter, and Collector)
The emitter is always drawn with the
arrow.
Figure 5-1. Perspective view of a silicon p-n-p bipolar transistor.
Differences between NPN & PNP
Type of
BJT
PNP-Type
NPN-Type
1
If the base is at a lower voltage than the emitter,
current flows from emitter to collector
If the base is at a higher voltage than the
emitter, current flows from collector to
emitter.
2
Small amount of current also flows from emitter to
base.
Small amount of current also flows from
base to emitter.
3
Emitter is heavily p-doped compared to collector. So,
emitter and collector are not interchangeable.
Emitter is heavily N-doped compared to
collector. So, emitter and collector are not
interchangeable.
4
The base width is small compared to the minority
carrier diffusion length. If the base is much larger,
then this will behave like back-to-back diodes.
The base width is small compared to the
minority carrier diffusion length. If the
base is much larger, then this will behave
like back-to-back diodes.
5
Voltage at base controls amount of current flow
through transistor (emitter to collector).
Voltage at base controls amount of current
flow through transistor (collector to
emitter).
Follow the arrow to see the direction of current flow
Follow the arrow to see the direction of
current flow
6
7
Operation of NPN Transistor
• In normal operation, the EB
junction is forward biased and
the BC junction is reverse
biased
• The base region is very thin
so the ratio L1:L2 is typically
about 150:1
Figure 5-2. (a) Idealized one-dimensional
schematic of a p-n-p bipolar transistor and
(b) its circuit symbol. (c) Idealized onedimensional schematic of an n-p-n bipolar
transistor and (d) its circuit symbol.
• The E is more heavily doped
than the C
• B doping is less than the E
doping but greater than the C
• At thermal equilibrium – no net I
flow  the Fermi level is a
constant
Figure 5-3. (a) A p-n-p transistor with all leads
grounded (at thermal equilibrium). (b) Doping profile
of a transistor with abrupt impurity distributions.
(c) Electric-field profile. (d) Energy band diagram at
thermal equilibrium.
BJT CONFIGRATIONS
Common-base configuration:
• Note: depletion layer width of the E-B
junction is narrower & C-B junction is
wider compared with equilibrium case
• E-B junction (forward biased) – holes
injected from the p+ E into B, electron
injected from the n B into E
• C-B junction (reverse biased) – if B
width is narrow, holes injected from the E
can diffuse thru B to reach the B-C
depletion edge and the “float up” into the
C
• E (emits/injects carriers)  C (collects
carriers from nearby junction) : C hole I 
E hole I
Figure 5.4.
• The transistor action: carriers injected
(a) The transistor shown in Fig. 3 under the from E junction  large I flow in C
active mode of operation.3 (b) Doping
junction
profiles and the depletion regions under
biasing conditions. (c) Electric-field profile.
(d) Energy band diagram.
Current gain
• Assume no generationrecombination Is in the depletion
regions
• Holes injected from E: IEp (largest I
component)
• Most of injected holes will reach C
junction - give rise ICp
• IBB : electrons that must be supplied
by the B to replace electrons
recombined with the injected holes
(IBB=IEp-ICp)
•IEn: I arising from electrons injected
from B to E – however not desirable
•ICn: thermally generated electrons
that are near the B-C junction edge
and drift from C to B – direction of
Figure 5.5. Various current components in a
electron I is opposite the electron
p-n-p transistor under active mode of
flow
operation. The electron flow is in the
opposite direction to the electron current.
Common-base current gain
Emitter efficiency
Base transport factor
Collector current

I Ep
IE
0 
I Cp

I Ep
T 
IE
I Ep  I En
I Cp
I Ep
I C   0 I E  I CBO
 0   T
ICBO : the leakage current
between the C and B with
the E-B junction open
Carrier Distribution
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To derive the I-V for an ideal transistor, assume:
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The device has uniform doping in each region
The hole drift current in the base region as well as the
C saturation current is negligible
There is low-level injection
There are no generation-recombination currents in
the depletion regions
There are no series resistance
Holes from E to B to C : Once the minoritycarrier distribution is determined (holes in the
n-type B)  can obtain I from the minoritycarrier gradient
pn (x): minority carrier (holes) in the base
pno: equilibrium minority carrier (holes) in
the base
nE: electron conc. in E
nC: electron conc. in C
nEO: equilibrium electron conc. in E
nCO: equilibrium electron conc. in C
Figure 5.6. Minority carrier distribution in various
regions of a p-n-p transistor under the active
mode of operation.
now look at what happens to the electrons injected into the base.
Because the base is made of p-type silicon, the electrons are minority
carriers. The base is very thin so the electron concentration, np, will
have a linear characteristic. The electron concentration will be
highest at the emitter side of the base, and will be zero at the
collector side. It is zero here because the CBJ is in reverse bias,
causing all minority carriers to be attached to and swept across to the
collector (by the same, majority carriers, holes, are repelled from the
junction). We will term the electron concentration at the EBJ np(0).
The EBJ is in forward bias, so the concentration at the emitter side of
the base np(0) will be proportional to evBE/vT:
Minority carrier in the base region:
p n  p no e
pno
e
qVEB / kT
qVEB / kT
x

1 
 W
x


  p n (0)1  

 W
: equilibrium minority-carrier
concentration in the base
: Exponential factor of the increased
minority carrier concentration at the edge of
the E-B depletion region (x=0)
Ideal Transistor Currents for Active Mode Operation:
I E  a11 (e
The emitter current
 D p pnO DE nEO 

a11  qA

W
L
E


qAD p p nO
The base current
a12 
 1)  a12
qAD p p nO
W
I C  a 21 (e qVEB / kT  1)  a 22
The collector current
a 21 
qVEB / kT
W
 D p p nO DC nCO
a 22  qA

LC
 W



I B  (a11  a21 )(e qVEB / kT  1)  (a12  a22 )
Note:
• Is in the 3 terminals are mainly determined by the minority carrier
distribution in the base region
• Common-base current gain 0 can be obtained
Modes of operation of p-n-p transistor
• Active mode:
• E-B junction is forward biased, B-C
junction is reverse-biased
• Saturation mode:
• both junctions are forward biased
• corresponds to small biasing V & large
output I – transistor is in a conducting
state & acts as a closed (or on) switch
• Cutoff mode:
• both junctions are reverse biased
• corresponds to the open (or off) switch
• Inverted mode:
Figure 5-7. Junction polarities and
minority carrier distributions of a p-n-p
transistor under four modes of
operation.
• inverted active mode
• E-B junction is reverse biased, C-B
junction is forward biased
I-V of Common-Base
• Ic is equal to IE (i.e 01) & independent of of VBC
• Ic remains constant even down to 0V for VBC (holes are still
extracted by C)
• Hole distributions (See fig. 5-9)
Figure 5-8. (a) Commonbase configuration of a p-n-p
transistor. (b) Its output
current-voltage
characteristics.
I-V of Common-Base
• Hole at x=W changes only
slightly from VBC>0 to VBC=0 (IC
remains the same) – fig. (a)
• to reduce IC to 0 – apply a
small forward bias (about 1V to
B-C junction) – fig. (b)
• The forward bias will increase
the hole density at x=W to make
it equal to that of the emitter at
x=0 (horizontal line)
• The hole gradient at x=W & IC
will reduce to 0
Figure 5.9. Minority carrier distributions in the base
region of a p-n-p transistor. (a) Active mode for VBC
= 0 and VBC > 0. (b) Saturation mode with both
junctions forward biased.
I-V of Common-emitter
IC 
0
I
I B  CBO
1 0
I  0
Common-emitter current gain:
0 
I C
0

I B 1   0
C-E leakage current: I CEO 
I CBO
1 0
I C   0 I B  I CEO
Figure 5.10. (a) Common-emitter config. of a p-n-p
transistor. (b) Its output I-V characteristics.
 0 Common-base current gain
Heterojunction bipolar transistor
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The heterojunction bipolar transistor (HBT) is an
improvement of the BJT that can handle signals of
very high frequencies up to several hundred GHz. It
is using mostly RF systems.
Heterojunction transistors have different
semiconductors for the elements of the transistor.
Usually the emitter is composed of a larger bandgap
material than the base. This helps reduce minority carrier
injection from the base when the emitter-base junction is
under forward bias and increases emitter injection
efficiency.
The improved injection of carriers into the base allows the
base to have a higher doping level, resulting in lower
resistance to access the base electrode.
Two commonly used HBTs are silicon–germanium and
aluminum gallium arsenide, though a wide variety of
semiconductors may be used for the HBT structure. HBT
structures are usually grown by epitaxy techniques like
MOCVD and MBE.
The Heterojunction Bipolar Transistor
• Emitter – wide band gap (AlGaAs)
• Base – lower band gap (GaAs)
• Large band gap difference (between
E-B)  common-emitter current gain
can be extremely large
• Homojunction: no band gap
difference – doping concentration in
the E & B must be very high
•EV increases the valence-band
barrier height  reduce injection of
holes from B to E
Figure 5-17. (a) Schematic cross
section of an n-p-n heterojunction
bipolar transistor (HBT) structure.
(b) Energy band diagram of a HBT
operated under active mode.
• can use heavily doped base,
maintain a high E efficiency & current
gain
Advanced HBTs
• InP-based material systems
• Advantages:
• very low surface recombination
• Higher electron mobility in
InGaAs than in GaAs – superior
high-freq performance (in fig.,
cutoff freq: 254GHz)
• InP collector region has higher
velocity at high fields than GaAs
collector
• InP collector breakdown voltage
is higher than GaAs
Figure 5-18. Current gain as a
function of operating frequency for
an InP-based HBT.
Cutoff freq., fT= 254GHz
Si/SiGe material system
• high-speed capability – because
the base is heavily doped (band
gap difference)
• Small trap density at Si surface
minimizes the surface
recombination current – high
current gain at low Ic
• Lower cutoff freq – because lower
mobility in Si compared to GaAs- &
InP- based HBTs
• Problem: E efficiency & Ic suffer
(caused by EV)
• To improve: graded-layer &
graded-base heterojunction
Figure 5.19.
(a) Device structure of an n-p-n Si/SiGe/Si HBT
(b) Collector and base current versus VEB for a HBT
and bipolar junction transistor (BJT).
• Wg: thickness of graded layer
• Graded profile in base region:
reduction of the band gap from E to C
• Ebi: built-in electric field:
• reduce minority-carrier transit
time
• increase the common-emitter
current gain
• increase the cutoff freq.
• Thicker C layer:
• improve breakdown voltage (BC junction)
• increase transit time
Figure 5.20. Energy band diagrams for a
heterojunction bipolar transistor with and
without graded layer in the junction, and with
and without a graded-base layer.
• Carrier move thru C at a saturation
velocity because of very large electric
fields are maintained in C
THYRISTOR & RELATED DEVICES
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Thyristor: designed for handling high V &
large I
Used for switching applications that require
the device to change from an off or
blocking state to an on conducting state
Thyristors have much wider range of I- & Vhandling capabilities
Available with I ratings from a few
miliamperes to over 5000A, V ratings
extending above 10,000V
• Called p-n-p-n diode
• Add gate electrode at the inner player (p2)  3-terminal device called
semiconductor-controlled rectifier
(SCR) or thyristor
Figure 5-22.
(a) Four-layer p-n-p-n diode. (b) Typical doping
profile of a thyristor. (c) Energy band diagram of
a thyristor in thermal equilibrium.
Basic I-V characteristics of p-n-p-n
diode – exhibits 5 distinct regions:
• 0-1: The device is in forward-blocking or
off-state and has very high impedance.
Forward breakover (switching) occurs
where dV/dI=0; & at point 1 defined as
forward breakover voltage VBF & switching
current IS
• 1-2: The device is in a negativeresistance region – the I increases as the V
decreases sharply
• 2-3: The device is in forward-conducting
or on-state & has low impedance. At point
2, where dV/dI=0, the holding current Ih &
holding voltage Vh
• 0-4: The device is in the reverse-blocking
state
Figure 5-23. Current-voltage
characteristics of a p-n-p-n diode.
• 4-5: The device is in the reversebreakdown region
Exercise 1
For an ideal p-n-p transistor , the current components are given by
IEp=3mA, IEn=0.01mA, ICp=2.99mA, and ICn=0.001mA. Determine:
(a) The emitter efficiency 
(b) The base transport factor T
(c) The common-bas current gain 0
(d) ICBO
Solution
(a) Emitter efficiency is:
 
I Ep
I Ep  I En

3
 0.9967
3  0.01
(b) The base transport factor :
T 
I Cp
I Ep

2.99
0.9967
3
(c) The common-base current gain
 0  T  0.9967  0.9967  0.9934
(d)
I E  I Ep  I En  3  0.01  3.01mA
I C  I Cp  I Cn  2.99  0.001  2.991
I CBO  I C   0 I E  2.991  0.9934  3.01  0.87A
Exercise 2
Referring to Exer. 1, find the common-emitter current
gain 0. Express ICEO in terms of 0 and ICBO and find the
value of ICEO
Solution
 The common-emitter current gain in Exer. 1 is 0.9934. Hence we
can obtain 0 by
0 
I CEO
0.9934
 150.5
1  0.9934
 0



 1 I CBO   0  1I CBO
1 0

I CEO  150.5  1  0.87  10 6  1.32  10 4 A