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4. Transistor Characteristics
Theory Support
Electronics - Diodes and Transistors
Transistor Characteristics
Topics covered in this presentation:
 Transistor Construction
 Transistor Operation
 Transistor Characteristics
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4. Transistor Characteristics
Theory Support
Electronics - Diodes and Transistors
The Transistor
The transistor is a semiconductor device that revolutionised electronics
in the 1950s and 60s.
Although individual transistors have been replaced in many
applications, transistors are still used in power gain applications.
They can be found in driver circuits where the signal from an input
or a process is not powerful enough to drive the output device.
There are many different types of transistors, but in this assignment we
will examine only the bipolar junction transistor.
Today, transistors are used in integrated devices where several
thousand transistors are formed in a single device.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Bipolar Junction Transistor
The bipolar junction transistor is a semiconductor
device containing two p-n junctions.
To form the two junctions, the
semiconductor material is
arranged in three layers.
This allows two different types
of transistor to be created,
depending on the order of the
n-type and p-type
semiconductor layers.
The two types of bipolar junction
transistors are NPN and PNP,
although only the NPN transistor
will be considered in this module.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Bipolar Junction Transistor
The three layers are called the base, emitter, and collector.
Base (B) - The base is the middle layer
of semiconductor material and controls
the flow of charge carriers between the
emitter and the collector.
Emitter (E) - The emitter region is
heavily doped with charge carriers
that can be emitted into the base.
Collector (C) - The collector is lightly
doped so that it can collect charge
carriers from the base region.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Bipolar Junction Transistor
Each type of bipolar junction
transistor has its own circuit
symbol showing the base,
emitter and collector.
The arrow on the symbol always
appears on the emitter terminal
and points from P to N.
For an NPN transistor, the arrow
on the emitter will point outwards.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
NPN Transistor Biasing
The bipolar junction transistor
contains two pn junctions that
behave exactly like a diode.
Each junction must be correctly
biased to allow electrons to flow
between the emitter and collector.
For this reason, an NPN
transistor cannot be replaced
by a PNP transistor.
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4. Transistor Characteristics
Theory Support
Electronics - Diodes and Transistors
NPN Transistor Biasing
For an NPN transistor, to allow the
electrons in the emitter to move to
the base, the voltage at the base
must typically be 0.6V higher than
the voltage at the emitter.
To allow these electrons to
continue on to the collector, the
collector voltage must be higher
than the base voltage.
Although some of the electrons
will exit the transistor at the base,
the majority of them will continue
onto the collector.
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4. Transistor Characteristics
Theory Support
Electronics - Diodes and Transistors
Transistor Characteristics
When selecting a transistor for an application, there are three
main characteristics that must be taken into account. These
are generally plotted as graphs.
Input Characteristic
• Base current (IB) against base-emitter voltage (VBE)
Output Characteristic
• Collector current (IC) against collector-emitter voltage (VCE)
Transfer Characteristic
• Collector current (IC) against base current (IB)
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Input Characteristic
The input characteristic is a plot of base current (IB) against
base-emitter voltage (VBE) for a fixed collector-emitter voltage.
From this graph you can determine what voltage must be applied
across the base and emitter in order to switch on the transistor.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Output Characteristic
The output characteristic is a plot of collector current (IC) against
collector-emitter voltage (VCE) for a range of base currents.
The graph shows a series of plots,
one for each base current value.
From the graph it can be seen
that initially the collector current
varies greatly for small change
to the collector emitter voltage.
At this point the transistor is not
very stable.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Output Characteristic
However, above a certain value the collector-emitter voltage has little
effect on the collector current (for a given base current) and the
transistor becomes more stable.
The transistor is generally
operated in this stable region, and
the output characteristic graph
shows what minimum value of
collector-emitter voltage must be
applied to keep it operating in the
stable region.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Transfer Characteristic
The transfer characteristic shows the collector current against base
current at a fixed value of collector-emitter voltage.
For a given collector-emitter voltage the graph will be approximately
linear, which shows that the base current is almost a constant
fraction of the collector current. The transistor acts as a current
amplifier.
The ratio of collector current to base
current is called the DC current gain (hFE).
DC current gain, hFE 
Collector current , IC
Base current , IB
The value of DC current gain varies between transistors
- even between transistors of the same type.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Transistor Datasheet
Some of the criteria, found on the product data sheet,
to consider when selecting a transistor.
•
•
•
•
•
DC current gain
Collector-emitter breakdown voltage
Collector-emitter saturation voltage
Base-emitter ‘on’ voltage
Package Type
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Transistor as a Switch
If the base-emitter voltage is below
0.6V, the transistor will not conduct,
resulting in no base current and no
collector current.
A base current is required to
obtain a collector current.
By switching the base current on
and off, the collector current can
also be switched on and off.
The transistor can therefore be
used as a switch with the base
current controlling the flow of the
collector current.
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4. Transistor Characteristics
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Electronics - Diodes and Transistors
Practical Transistor Switch Circuit
In a practical transistor switch
circuit the load is connected
between the collector and the
positive supply rail.
A resistor is connected between
the base and the power supply.
This is to ensure that the current
through the base-emitter junction
is kept to a safe level, in order to
prevent damage to the transistor.
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