COURSE NAME: SEMICONDUCTORS
Course Code: PHYS 473
Week No. 8
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Transistor
• A three lead semiconductor device that acts as:
(1)
(2)
an electrically controlled switch
a current amplifier.
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Bipolar Junction Transistor (BJT)
A bipolar junction transistor (BJT) is a type of transistor that
relies on the contact of two types of semiconductor for its
operation.
• A transistor consisting of two n- and one p-type layers of
material called NPN transistor.
• A transistor consisting of two p- and one n-type layers of
material called PNP transistor.
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Basic Models of BJT
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Bipolar Junction Transistor Fundamentals
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Basic principle of the BJT
The basic principle of the BJT is:
“The voltage between two terminals controls the current through
the third terminal”.
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Working of NPN transistor.
• Figure shows the NPN transistor with forward bias to emitter
base junction and reverse bias to collector-base junction.
• The forward bias causes the electrons in the n-type emitter to
flow towards the base. This constitutes the emitter current IE.
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Working of NPN transistor.
• As these electrons flow through the p-type base, they tend to
combine with holes.
• As the base is lightly doped and very thin, therefore, only a few
electrons (less than 5%) combine with holes to constitute base
current IB.
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Working of NPN transistor.
• The remainder (more than 95%) cross over into the collector
region to constitute collector current IC.
• In this way, almost the entire emitter current flows in the
collector circuit.
• It is clear that emitter current is the sum of collector and base
currents i.e
IE = IB + IC
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Working of PNP transistor.
• The forward bias causes the holes in the p-type emitter to flow
towards the base. This constitutes the emitter current IE.
• As these holes cross into n-type base, they tend to combine with
the electrons.
• As the base is lightly doped and very thin, therefore, only a few
holes (less than 5%) combine with the electrons.
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Working of PNP transistor.
• The remainder (more than 95%) cross into the collector region
to constitute collector current IC.
• In this way, almost the entire emitter current flows in the
collector circuit.
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Conventional currents
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Conventional current flow (NPN)
For NPN connection, it is clear that conventional current flows
out of the emitter as indicated by the outgoing arrow in Fig.
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Conventional current flow (PNP)
For PNP connection, it is clear that conventional current flows
into the emitter as indicated by the inward arrow in Fig.
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Types of configurations
• Common Base Configuration (CBC):
• Common Emitter Configuration (CEC):
• Common Collector Configuration (CCC):
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Common Base Configuration (CBC)
Current amplification factor (α).
• The ratio of change in collector current to the change in emitter
current at constant collector base voltage VCB is known as
current amplification factor i.e.
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Characteristics of Common Base Connection
Input characteristic.
1.The emitter current IE increases
rapidly with small increase in
emitter-base voltage VEB.
It means that input resistance is
very small.
2.The emitter current is almost
independent of collector-base
voltage VCB.
This leads to the conclusion that
emitter
current
is
almost
independent of collector voltage.
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Characteristics of Common Base Connection
Output characteristic.
1.The collector current IC varies
with VCB only at very low
voltages ( < 1V).
The transistor is never operated in
this region.
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Characteristics of Common Base Connection
Output characteristic.
2.When the value of VCB is raised
above 1 − 2 V, the collector current
becomes constant as indicated by
straight horizontal curves.
It means that now IC is independent
of VCB and depends upon IE only.
The transistor is always operated in
this region.
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Characteristics of Common Base Connection
Output characteristic.
3. A very large change in collectorbase voltage produces only a tiny
change in collector current.
This means that output resistance
is very high.
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Common Base Configuration (CBC)
Q: 1
Q: 2
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Common Base Configuration (CBC)
Q. 3
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Common Base Configuration (CBC)
• Q. 4
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Common Emitter Connection
Base current amplification factor (β). The ratio of change in
collector current (ΔIC) to the change in base current (ΔIB) is
known as base current amplification factor i.e.
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Common Emitter Connection
Q. 5
Q. 6
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Common Emitter Connection
Q. 7
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Common Emitter Connection
Q. 8
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Characteristics of Common Emitter Connection
Input characteristic.
As compared to CB arrangement,
IB increases less rapidly with VBE.
Therefore, input resistance of a CE
circuit is higher than that of CB
circuit.
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Characteristics of Common Emitter Connection
Output characteristic.
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Characteristics of Common Emitter Connection
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Characteristics of Common Emitter Connection
1. Cutoff region:
Base-emitter junction is reverse biased.
No current flow.
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Characteristics of Common Emitter Connection
2. Saturation region:
• Base-emitter junction forward biased.
• Collector-base junction is forward
biased.
• IC reaches a maximum which is
independent of IB and β.
• VCE < VBE
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Characteristics of Common Emitter Connection
3. Active region:
• Base-emitter junction forward biased
• Collector-base junction is reverse
biased
• VBE < VCE < VCC
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Characteristics of Common Emitter Connection
4. Breakdown region:
• IC and VCE exceed specifications
• Damage to the transistor
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Common Collector Connection
Current amplification factor γ.
The ratio of change in emitter current (ΔIE) to the change in base
current (ΔIB) is known as current amplification factor in
common collector (CC) arrangement i.e.
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Types of configurations
• Common Base Configuration (CBC):
• Common Emitter Configuration (CEC):
• Common Collector Configuration (CCC):
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BJT relationships-Equations
IE = I C + IB
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Example No. 1
In the given figure, if IB = 50 A, IC = 1mA, then find the
followings: 1) IE
2) 
3) 
Solution:
1) Since: IE = IB + IC
IE = 50  A + 1 mA
IE = 0.05 mA + 1 mA
IE = 1.05 mA
2) Since:  = IC/IB
 = 1 mA / 0.05 mA = 20
 = 20
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Example No. 1 continued…………
Solution:
3) Since  = IC / IE = 1 mA / 1.05 mA = 0.95238
 = 1 mA / 1.05 mA
 = 0.95238
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Example No. 2
An NPN Transistor has a DC current gain, (Beta) value of 200.
Calculate the base current IB required to switch a resistive load of
4mA.
Solution:
Given :   200
I C  4mA  4 103 A
IB  ?
IC
current gain= 
IB
4 103 A 4 103 A 4 103 A
 IB 




200
200
2 102
IC
 2 103 2 A  2 105 A  20 106 A
 20 A 1  106
 I B  20 A
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Example No. 3
If the collector current IC=7.95 mA and the emitter current IE =8mA,
then calculate current gain alpha.
Solution:
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Modes of Operation
• Depending on the bias condition on its two junctions, the BJT
can operate in one of three possible modes:
• cut-off (both junctions reverse biased)
• active (the EBJ forward-biased and CBJ reversed)
• saturation (both junctions forward biased)
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Advantages of transistors
High voltage gain.
2. Lower supply voltage.
3. No heating.
4. Mechanically strong due to solid-state
1.
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Disadvantages of transistors
Lower power dissipation
2. Lower input impedance
3. Temperature dependence
4. Inherent variation of parameters
1.
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Summary of Bipolar Junction Transistors
• BJT is a three layer device constructed form two semiconductor
diode junctions joined together, one forward biased and one reverse
biased.
• There are two main types of BJT, the NPN and the PNP transistor.
• Transistors are "Current Operated Devices" where a much smaller
Base current causes a larger Emitter to Collector current, which
themselves are nearly equal, to flow.
• The most common transistor connection is the Common-emitter
configuration.