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Chapter 5
© M.N.A. Halif & S.N. Sabki
The Bipolar Junction Transistor
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
© M.N.A. Halif & S.N. Sabki
Differences between NPN & PNP
Type of
BJT
PNP-Type
NPN-Type
1
PNP. If the base is at a lower voltage than the emitter,
current flows from emitter to collector
NPN. If the base is at a higher voltage than
the emitter, current flows from
collector to emitter.
2
PNP. Small amount of current also flows from
emitter to base.
NPN. 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
PNP. Voltage at base controls amount of current flow
through transistor (emitter to collector).
NPN. 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
© M.N.A. Halif & S.N. Sabki
Operation of NPN Transistor
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.
© M.N.A. Halif & S.N. Sabki
• 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.
© M.N.A. Halif & S.N. Sabki
© M.N.A. Halif & S.N. Sabki
BJT CONFIGRATIONS
© M.N.A. Halif & S.N. Sabki
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.
(a) The transistor shown in Fig. 3 under
the active mode of operation.3 (b) Doping
profiles and the depletion regions under
biasing conditions. (c) Electric-field
profile. (d) Energy band diagram.
• The transistor action: carriers injected
from E junction  large I flow in C
junction
© M.N.A. Halif & S.N. Sabki
© M.N.A. Halif & S.N. Sabki
© M.N.A. Halif & S.N. Sabki
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
© M.N.A. Halif & S.N. Sabki
© M.N.A. Halif & S.N. Sabki
Carrier Distribution
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 wil 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
© M.N.A. Halif & S.N. Sabki
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-np transistor under four modes of
operation.
• inverted active mode
• E-B junction is reverse biased, C-B
junction is forward biased
© M.N.A. Halif & S.N. Sabki
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-np transistor. (b) Its output
current-voltage
characteristics.
© M.N.A. Halif & S.N. Sabki
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.
© M.N.A. Halif & S.N. Sabki
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
© M.N.A. Halif & S.N. Sabki
Example
A bipolar transistor with an emitter
current of 1 mA has an emitter
efficiency of 0.99, a base transport
factor of 0.995 and a depletion layer
recombination factor of 0.998
Calculate the transport factor.
The transport factor and current gain are
© M.N.A. Halif & S.N. Sabki
Heterojunction bipolar transistor
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.
© M.N.A. Halif & S.N. Sabki
The Heterojunction Bipolar Transistor
• Emitter – wide bandgap (AlGaAs)
• Base – lower bandgap (GaAs)
• Large bandgap difference (between
E-B)  common-emitter current gain
can be extremely large
• Homojunction: no bandgap
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
© M.N.A. Halif & S.N. Sabki
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
© M.N.A. Halif & S.N. Sabki
Si/SiGe material system
• high-speed capability –
because the base is heavily
doped (bandgap difference)
• Small trap density at Si surface
minimizes the surface
recombination current – high
current gain at low Ic
• Lower cutoff freq – because lower
mobilities 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).
© M.N.A. Halif & S.N. Sabki
THYRISTOR & RELATED DEVICES
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
© M.N.A. Halif & S.N. Sabki
• 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.
© M.N.A. Halif & S.N. Sabki
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
© M.N.A. Halif & S.N. Sabki