Presentation
On
BIPOLAR JUNCTION TRANSISTOR
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Introduction
• A bipolar junction transistor (BJT) is a three terminal semiconductor
device in which the operation depends on the interaction of both
majority and minority carriers and hence the name Bipolar.
• The BJT is analogous to a vacuum triode and is comparatively
smaller in size.
• It is used in amplifier and oscillator circuits, and as a switch in
digital circuit.
• It has wide applications in computers, satellites and other modern
communication system.
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Construction
• The BJT consists of a silicon (or germanium) crystal
in which a thin layer of N-type silicon is sandwiched
between two layers of P-type silicon.
• This transistor is referred to as PNP. Alternatively, in
a NPN transistor, a layer of P-type material is
sandwiched between two layers of N-type material.
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NPN and PNP
B
E
N
P
B
N
C
E
NPN
P
N
P
C
PNP
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• The three portions of the transistor are Emitter, Base and Collector,
shown as E, B, and C, respectively.
• The arrow on the emitter specifies the direction of current flow
when the EB junction is forward biased.
• Emitter is heavily doped so that it can inject a large number of
charge carriers into the base.
• Base is lightly doped and very thin.
• It passes most of the injected charge carriers from the emitter into
the collector.
• Collector is moderately doped.
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Symbol
C
E
B
Symbol of NPN transistor
E
C
B
Symbol of PNP transistor
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Transistor biasing
• The following figure shows, usually the emitter-base
junction is forward biased and collector-base junction is
reverse biased.
• Due to the forward bias on the emitter-base junction, an
emitter current flows through the base into the collector.
• Through the collector-base junction is reverse biased,
almost the entire emitter current flows through the collector
circuit.
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NPN Transistor biasing
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PNP Transistor biasing
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Active-mode NPN transistors in circuits
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• The diagram opposite is a schematic representation of an NPN transistor
connected to two voltage sources. To make the transistor conduct
appreciable current (on the order of 1 mA) from C to E, VBE must be above
a minimum value sometimes referred to as the cut-in voltage.
• The cut-in voltage is usually about 600 mV for silicon BJTs at room
temperature but can be different depending on the type of transistor and its
biasing.
• This applied voltage causes the lower P-N junction to 'turn-on' allowing a
flow of electrons from the emitter into the base. In active mode, the electric
field existing between base and collector (caused by VCE) will cause the
majority of these electrons to cross the upper P-N junction into the collector
to form the collector current IC.
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• The remainder of the electrons recombine with holes, the majority carriers
in the base, making a current through the base connection to form the base
current, IB.
• As shown in the diagram, the emitter current, IE, is the total transistor
current, which is the sum of the other terminal currents (i.e., IE = IB + IC ).
• The arrows representing current point in the direction of conventional
current – the flow of electrons is in the opposite direction of the arrows
because electrons carry negative electric charge.
• In active mode, the ratio of the collector current to the base current is called
the DC current gain.
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• This gain is usually 100 or more, but robust circuit
designs do not depend on the exact value (for
example - op-amp).
• The value of this gain for DC signals is referred to as
hFE, and the value of this gain for AC signals is
referred to as hfe.
• However, when there is no particular frequency range
of interest, the symbol β is used
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Active-mode PNP transistors in circuits
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• The diagram opposite is a schematic representation of a PNP transistor
connected to two voltage sources. To make the transistor conduct
appreciable current (on the order of 1 mA) from E to C, VEB must be above
a minimum value sometimes referred to as the cut-in voltage.
• The cut-in voltage is usually about 600 mV for silicon BJTs at room
temperature but can be different depending on the type of transistor and its
biasing.
• This applied voltage causes the upper P-N junction to 'turn-on' allowing a
flow of holes from the emitter into the base.
• In active mode, the electric field existing between the emitter and the
collector (caused by VCE) causes the majority of these holes to cross the
lower P-N junction into the collector to form the collector current IC.
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• The remainder of the holes recombine with electrons, the majority
carriers in the base, making a current through the base connection to
form the base current, IB.
• As shown in the diagram, the emitter current, IE, is the total
transistor current, which is the sum of the other terminal currents
(i.e., IE = IB + IC).
• The arrows representing current point in the direction of
conventional current – the flow of holes is in the same direction of
the arrows because holes carry positive electric charge. In active
mode, the ratio of the collector current to the base current is called
the DC current gain.
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• This gain is usually 100 or more, but robust
circuit designs do not depend on the exact value.
• The value of this gain for DC signals is referred to
as hFE, and the value of this gain for AC signals is
referred to as hfe.
• However, when there is no particular frequency
range of interest, the symbol β is used
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Types of configuration
• When a transistor is to be connected in a circuit, one terminal is used
as an input terminal, the other terminal is used as an output terminal
and the third terminal is common to the input and output.
• Depending upon the input, output and common terminal, a transistor
can be connected in three configurations.
– Common base configuration
– Common emitter configuration
– Common collector configuration
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CB Configuration
• This
is
also
called
grounded
base
configuration. In this configuration, emitter is
the input terminal, collector is the output
terminal and base is the common terminal.
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NPN common-base circuit
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CE Configuration
• This
is
also
called
grounded
emitter
configuration. In this configuration, base is
the input terminal, collector is the output
terminal and emitter is the common terminal.
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NPN common-emitter circuit
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CC Configuration
• This
is
also
called
grounded
collector
configuration. In this configuration, base is the
input terminal, emitter is the output terminal
and collector is the common terminal.
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NPN common-collector circuit
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Comparison of CB CE and CC configuration
S .No
Characteristics
CB
CE
CC
1
Input impedance
Low
Medium
High
2
Output impedance
High
medium
low
3
Current gain
Low
High
High
4
Voltage gain
High
High
Unity
5
Power gain
Medium
High
Low
6
Phase reversal
No
Yes
No
7
application
AF amplifiers
Voltage &
power
amplifiers
Impedance
matching
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Transistor as a switch
• When a transistor is used as a switch, it is usually required
to be brought alternatively in the saturation and cut-off
conditions.
• When in saturation condition, it should carry heavy current,
so the voltage drop across the transistor is as near to zero as
possible. It may be considered as “closed switch”.
• When in cut-off condition, it should carry almost no current
so that it may be considered to be an “open switch”.
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Applications
• The BJT remains a device that excels in some applications, such as
discrete circuit design, due to the very wide selection of BJT types
available, and because of its high transconductance and output
resistance compared to MOSFETs.
•
The BJT is also the choice for demanding analog circuits, especially
for very-high-frequency applications, such as radio-frequency
circuits for wireless systems. Bipolar transistors can be combined
with MOSFETs in an integrated circuit by using a BiCMOS process
of wafer fabrication to create circuits that take advantage of the
application strengths of both types of transistor.
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Temperature sensors
• Because of the known temperature and current
dependence of the forward-biased base–emitter
junction voltage, the BJT can be used to measure
temperature by subtracting two voltages at two
different bias currents in a known ratio
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Logarithmic converters
• Because base–emitter voltage varies as the log of
the base–emitter and collector–emitter currents, a
BJT can also be used to compute logarithms and
anti-logarithms.
• A diode can also perform these nonlinear
functions, but the transistor provides more circuit
flexibility.
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Vulnerabilities
• Exposure of the transistor to ionizing radiation causes radiation
damage. Radiation causes a buildup of 'defects' in the base region
that act as recombination centers.
• The resulting reduction in minority carrier lifetime causes gradual
loss of gain of the transistor.
• Power BJTs are subject to a failure mode called secondary
breakdown, in which excessive current and normal imperfections in
the silicon die cause portions of the silicon inside the device become
disproportionately hotter than the others.
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• The doped silicon has a negative temperature coefficient, meaning
that it conducts more current at higher temperatures.
• Thus, the hottest part of the die conducts the most current, causing
its conductivity to increase, which then causes it to become
progressively hotter again, until the device fails internally.
•
The
thermal
runaway
process
associated
with
secondary
breakdown, once triggered, occurs almost instantly and may
catastrophically damage the transistor package.
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The End
M.S.P.V.L Polytechnic College,
Department of ECE,
Pavoorchatram.
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