Introduction to Transistors
Transistors come in two general types. The first is the Bipolar Junction Transistor (BJT) which is the topic
of the material that follows. The second type is the Field Effect Transistor (FET) which will be looked at
in another part of the course.
Bipolar Junction Transistor (BJT)
This transistor is a semiconductor device consisting of 2 P N junctions – the Base-Emitter junction and
the Base-Collector junction. There are two different types of BJT referred to as NPN and PNP transistors.
Diagrams illustrating their construction are shown.
NPN Transistor
As the name suggests this transistor is constructed from 2 sections of N type semiconductor material
and 1 of P material creating 2 PN junctions.
PNP Transistor
This transistor also has 2 PN junctions but is constructed from 2 sections of P type semiconductor
material and 1 of N material.
What is the Difference?
The most significant difference between these 2 types of transistors relates to how they are used. NPN
transistors are used with positive supply voltages called +VCC and the PNP uses a negative supply voltage
referred to as –VCC. We will focus almost exclusively on the NPN type of transistor.
Transistor Terminals
A BJT transistor has 3 leads that are referred to as the Base (B), Collector (C) and Emitter (E). Both the
physical and schematic symbols below show these terminals.
Transistor Schematic Symbols
The standard symbols for use in a schematic diagram are shown.
A slightly different approach to viewing these symbols is shown as well. These alternate diagrams are
consistent with the idea that the terminal pointing “up” is usually connected to the supply voltage
(either +VCC or –VCC) and the terminal pointing “down” is connected to ground.
Transistor Electron Flow
The 3 regions of the transistor are not manufactured exactly the same. The emitter area is a large
heavily doped N type region with lots of free electrons, the base is a small, thin lightly doped (low
density of holes) P type area and the collector is a large moderately doped N type area. An NPN
transistor is shown with 2 bias voltages VCC and VBB.
The supply voltage VCC has its negative terminal connected to the Emitter. This will repel free electrons
into the Emitter region and through to the lightly doped base region where a small percentage of these
electrons will combine with holes and flow out of the base terminal by the attraction of the VBB voltage
as valence electrons. The rest of the electrons injected the base will continue on into the Collector
region creating a large electron flow from Emitter to Collector in the transistor.
We can attempt to quantify the number of electrons moving through each region of the transistor. If
the assumption is that 100 electrons flow through the emitter into the base, the design of the transistor
is such that approximately 1 electron will flow out of the base and the other 99 will continue to the
collector region.
Transistor Conventional Current Flow
Conventional current as previously learned flows in the opposite direction to electron flow. The diagram
below shows the 3 transistor currents; Base Current (IB), Collector current (IC) and Emitter current (IE)
with their correct direction of flow.
Transistor Current Relationships
From the diagram above its can be written that
IE = IB + IC
This is just an application of KCL. From the description and quantification of electron flow above it can
be predicted that relatively speaking the Base current in a transistor is small and the Collector and
Emitter currents are large.
So the expression above can often be approximated as
IE ≈ IC
Transistor Current Block Diagram
From a current point of view the block diagram represents the input and output currents of the
transistor.
The Base current IB is the Input current and the Collector current IC is the Output current.
Transistor Parameters
There are a number of transistor parameters that can be considered.
Current Gain - β
The Current Gain - β - is defined as the ratio of Collector to Base currents. β of course has no units.
β = IC/IB
From this definition we can write that
IC = β x IB
The outcome of this expression is important. It tells us that the collector current is β times larger than
the base current.
In fact it can be written that the smaller Input Base current IB controls the larger Output Collector
current IC .
It is often written that the BJT is a current controlled device – “an input current controls an output
current.”
From the expression IC = β x IB the following can be derived
IE = (β + 1) x IB
Variability of Current Gain - β
Values of β can be measured or found on a data sheet for a transistor. A more advanced term hFE is
also used to describe current gain. However for any given transistor the value of β is highly variable.
In what ways does it vary?
1. β varies from device to device. For example:
A 2N 4124 is a small signal transistor. From the data sheet the value of β varies from 120 to 360. A
diagram of this transistor is shown with a TO-92 casing.
A 2N 3055 is a power transistor. From the data sheet its β varies from 20 to 70. Diagrams of this
transistor are shown with a TO-3 and a TO-220 casing.
2. β also varies within devices as can be seen above. There is at least
a 100 % variation in the values of β for these devices.
3. β also varies with the value of collector current.
4. β also varies with temperature.
Alpha Parameter – α
The Alpha parameter is the ratio of the number of electrons that leave the Collector region to the
number of electrons that enter the Emitter. So in terms of current flow
α = IC/IE
From this expression it can be shown that
IC = α IE
and
IB = IE - IC = IE - α IE
so
IB = (1 – α) IE
Also α and β can be related
α = β/(β + 1)
and
β = α/(1 - α)
Example
1. If IE = 20 mA and β = 200. Find IB, IC, and α
IB = IE/(β + 1) = 20 mA/(200 +1) = 100 µA
IC = IE - IB = 20 mA – 0.1 ma = 19.9 mA
α = IC/IE = 19.9 mA/20 mA = 0.995
2. If IC = 50 mA and β = 400. Find IB, IE, and α
IB = IC/β = 50 mA/400 = 125 µA
IE = IC + IB = 50 mA + 0.125 ma = 50.125 mA
α = IC/IE = 50 mA/50.125 mA = 0.996
Note: the value of α for this question indicates also that if 1000 electrons enter the emitter
region, 4 leave the base region and 996 continue to the collector.
Transistor Testing
Transistors can be tested using an Ohmmeter in a similar way to testing a diode.
Back to Back Diode Model of Transistor
This model is helpful to see the 2 PN junctions in the transistor and how they can be tested.
NPN Transistor
PNP Transistor
For this simple model the transistor is tested as if it was a pair of diodes. The Base Emitter
junction is one diode and the Base Collector junction is the other. Use the ohmmeter and
connect the leads to each anode and cathode of the equivalent PN junctions. The results for
working transistor are shown.
Polarity of Connection
On or Off
Resistance
B (+)
E (-)
On
Low R
B (-)
E (+)
Off
High R
B (+)
C (-)
On
Low R
B (-)
C (+)
Off
High R
C (+)
E (-)
Off
High R
C (-)
E (+)
Off
High R
PNP Transistor Testing
The testing for a PNP transistor is very similar. The table shows the results for a functional
transistor.
Polarity of Connection
On or Off
Resistance
B (+)
E (-)
Off
High R
B (-)
E (+)
On
Low R
B (+)
C (-)
Off
High R
B (-)
C (+)
On
Low R
C (+)
E (-)
Off
High R
C (-)
E (+)
Off
High R