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The transistor
In 1948 some work was carried out at the Bell Telephone Laboratories in America that has
changed our lives. This was the invention of the transistor by Shockley, Brattain and
Bardeen.
The transistor, basically a semiconductor triode, consists of a thin central layer of one type of
semiconductor between two relatively thick pieces of the other type. The junction transistor
can be of two types, as shown in Figure 1: pnp or npn. The pnp transistor consists of a very
thin piece of n-type material sandwiched between two pieces of p-type, while the npn
transistor has a central piece of p-type. The pieces at either side are called the emitter and
the collector while the central part is known as the base. The base is lightly doped compared
with the emitter and the collector, and is only about 3-5 m thick.
n
base
collector
collector
p
p
n
n
p
collector
emitter
base
npn
base
emitter
emitter
Figure 1
pnp
From now on we will consider only the npn transistor as it is now in more common use in
schools. The npn silicon transistor is connected into the circuit as shown by Figure 2. The
emitter-base junction is forward biased and the base-collector junction is reverse biased.
R
IC
IB
electron flow
IE
Figure 2
When the base-emitter voltage is 0.6 V current will flow through the transistor, electrons
flowing through the base from the emitter to the collector. No current will flow without this
base-emitter voltage, since it is needed to overcome the potential barrier formed at the
junction. Electrons flow into the collector, although the base-collector junction is reverse
biased because the base is very thin.
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You should see that the emitter current (IE) is the sum of the base current (IB) and the
collector current (IC):
Emitter current = Base current + Collector current
IE = IB + IC
The collector current (IC) is usually over 99 per cent of IE and IB is less than 1 per cent.
The name ‘transistor’ comes from the words ‘transfer of resistance’: the emitter-base junction
is forward biased and therefore has a low resistance, while the base-collector is reverse
biased and has a high resistance.
The properties of the transistor described above lead us to consider it as a current amplifier.
mA
Student investigation
A very small current, known as a leakage current
will flow through the transistor even if there is no
supply to the base. This experiment is designed
to investigate how the leakage current varies with
temperature.
Set up the circuit as shown in Figure 3, taking
care not to let the transistor leads get in the
water. Slowly heat the water and record a set of
values of leakage current against temperature. Be
sure to connect the transistor with the correct
polarity.
Suggest why the current varies in the way you
have observed.
Plot a graph of your results and then use the
transistor to measure the temperature of your
hand.
cork
E
C
npn transistor
water
Figure 3
Example circuits with the npn transistor
For its basic operation, the circuit is set up as shown in Figure 4, and the value of R is
chosen so that the transistor is switched on, that is, the potential at the base is at least 0.6 V.
Lamp L2 lights but L1 does not, showing
that the collector current must be much
larger than the base current.
If L1 is removed, however, L2 goes out
because no potential is being applied to
the base.
L2
R
6V
L1
Figure 4
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The transistor as a switch
A transistor will not conduct (that is, no current will flow through from the collector to the
emitter) unless there is a also potential difference between the emitter and the base of at
least 0.6 V. This property enables the transistor to be used as a switch: it is ‘on’ when the
base-emitter potential difference is bigger than 0.6 V and ‘off’ when it isn’t.
If you consider the circuit in Figure 5,
then when the transistor is off, that is,
there is no current flowing through it, the R1
potential difference across the emittercollector (VCE) is high.
As soon as the transistor starts to
conduct this potential difference falls to
very close to zero (Figure 6).
Therefore the output potential difference
(VCE) is small when the input potential
difference (VBE) is large, and large when
the input potential difference is small,
that is less than 0.6 V.
This is the basic NOT logic gate circuit. (see the
section on Logic gates in the Foundation level for
a further treatment).
R2
VCE
VBE
Figure 5
VCE
We will now consider two circuits in which the
switching action of a transistor is important.
Figure 6
(a) Making a light come on in the dark
VBE
The circuit includes a light-dependent resistor
(LDR), the resistance of which changes with
illumination. A table showing the variation for a light-dependent resistor in common use is
shown below.
Conditions
Darkness
60W buIb at 1m
1 W buIb at 0.1 m
Fluorescent lighting
Bright sunlight
Resistance
> 10 M
2.4 k
1.1 k
275 
10 
The circuit used is shown in Figure 7. Since the
resistance of the light-dependent resistor varies so
will the voltage drop across it, and therefore the
potential at the base will change. The less light that
shines on the LDR the higher its resistance, and
therefore the larger VBE will be. If this is above 0.6 V
the transistor switches on, and so when the LDR is in
darkness the transistor conducts and the lamp L
comes on.
schoolphysics 16-19/Electronics/Transistors/Text/Transistor
L
R
VBE
LDR
Figure 7
3
(b) Moisture detector
The circuit is shown in Figure 8. If
the base circuit is broken at XY then
the transistor is off, but if the probes X
XY are placed in a conducting liquid
the transistor switches on. This Y
could be used as a liquid level
indicator for a blind person, the
lamp L being replaced by a buzzer
and the two contacts being placed
at a suitable level in a cup or bowl.
L
R
Figure 8
Saturation
We have seen that when VBE >0.6 V VCE (V)
6
the transistor switches on, and Figure
9 shows that as VBE is increased
above this value VCE falls and reaches
a steady value (close to zero) when
VBE is about 1.4 V. Any further
increase in VBE does not change VCE.
In this condition the transistor is said
to have bottomed or be saturated.
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 V (V)
BE
Figure 9
Student investigation
Design and build a simple fire alarm based on the switching action of a transistor and
using a thermistor as a heat sensor. Explain how you would be able to vary the
temperature at which the alarm is triggered.
Student investigation
Design and build a circuit based on the switching action of a transistor that will close the
contacts of a relay after a certain time. (Note that a diode should be connected in parallel
with the relay to protect the transistor from the large e.m.f. induced in the relay coil when
the current falls to zero when the circuit is switched off.)
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