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The
transistor
was developed in 1948. |
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The transistor was initially developed by a small team led by William Shockley. Their work on semiconductors was recognised to be very important and they received the Nobel prize for this in 1956. |
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The transistor was to replace the valve. Older products using valves (such as the radio on the right), were much larger, more fragile and less reliable than products we use today. |
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In the early 1950's, small pocket-sized transistor radios like these were developed. The transistor has almost completey replaced the valve now, with the exception of some guitar amplifier manufacturers wishing to retain the earlier sound the valve produced.
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An older valve and the |
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Semiconductor/transistor design is at the heart of the technology we use in computers today - where many millions of microscopic transistors are embedded into computer processors (as shown on the right). |
The bipolar transistor uses two special types of material
called n-type and p-type
semiconductors. These have
special properties which makes them part conductor and part
insulator. Being "semi-conductors" they can either allow current to flow or not to flow - which is very useful.
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To make a transistor, three layers of n-type
and
p-type semiconductors are needed. They can
either be
sandwiched together to make npn or pnp type transistors.
The transistors shown below are the npn type. These are
very common. The connections to the layers, or legs of the
transistor, are known as the emitter, base and collector.
The current needed to make the lamp work can be
reasonably large. When we turn a torch switch to "on" the
required current will flow from the battery, through the switch
and then make the lamp shine.The switch can make this happen as
its contacts inside are made of a very good conducting material - sometimes they are gold plated.
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Using the following
block diagrams we can now look at how a
transistor can be put into the system to act as an electronic
switch.
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Block diagram of transistor controller |
In the circuit below we will see how the transistor switch is made to turn on
when a small current (shown green) flows into the base of the transistor, via a switch and current limiting resistor.
The output lamp
switches on now as the larger current
(shown blue) is allowed to flow into the collector and through the transistor.
The two currents entering the transistor combine and flow out of the emitter and back to the negative side of the battery to complete the circuit.
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When the switch is opened, the small input current no
longer flows into the transistor base connection. This switches the transistor off again, stopping the larger output current flowing through the transistor. This causes the lamp to turn off.
Additional Points:
1, The resistor
in the circuit is used to reduce the current
flowing into the transistor - without it the transistor would
be damaged. This is similar to the reason a protection resistor is needed by an L.E.D.
2,In control systems, transistors can either be used to make sensitive electronic switches or they can be used as a "buffer" to provide higher power to an output device when operated, or switched, by a lower current input device.