COURSE NAME: SEMICONDUCTORS
Course Code: PHYS 473
Week No. 5
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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.
(1) NPN
(2) PNP
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BJT types
• 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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Bipolar Junction Transistor Fundamentals
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Types of configurations
• Common Base Configuration:
has Voltage Gain but no Current Gain.
• Common Emitter Configuration:
has both Current and Voltage Gain.
• Common Collector Configuration:
has Current Gain but no Voltage Gain.
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Types of configurations
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NPN BJT: How it works
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NPN Transistor in a Complete Circuit
(1)
(2)
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A simplified structure of the npn transistor
• Transistor consists of two pn-junctions:
• emitter-base junction (EBJ)
• collector-base junction (CBJ)
• Operating mode depends on biasing.
• active mode – used for amplification
• cutoff and saturation modes – used for switching.
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Current Flow
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Current Flow
Forward bias on emitter-base junction will cause current to flow.
This current has two components:
• electrons injected from emitter into base
• holes injected from base into emitter.
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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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Basic formulae of BJT
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Question No. 1
An NPN Transistor has a DC current gain, (Beta) value of 200.
Calculate the base current ( I B ) required to switch a resistive load of
4mA.
Solution:
IC
Given :   200
current gain= 
IB
I C  4mA  4 103 A
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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Question No. 2
If the collector current Ic = 7.95 mA and the emitter current
Ie=8mA, then calculate current gain alpha.
Solution:
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Input characteristics of NPN transistor
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Transistor Operation and Characteristic I-V curves
The three terminals of the transistors and the two junctions,
present us with multiple operating regimes.
In order to distinguish these regimes we have to look at the I-V
characteristics of the device.
The most important characteristic of the BJT is the plot of the
collector current, Ic , versus the collector-emitter voltage, VCE, for
various values of the base current, IB as shown on the circuit.
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Transistor Operation and Characteristic I-V curves
Cutoff region:
Base-emitter junction is reverse biased. No current flow.
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Transistor Operation and Characteristic I-V curves
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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Transistor Operation and Characteristic I-V curves
Active region:
• Base-emitter junction forward biased
• Collector-base junction is reverse
biased
• VBE < VCE < VCC
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Transistor Operation and Characteristic I-V curves
Breakdown region:
• IC and VCE exceed specifications
• damage to the transistor
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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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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.