TRANSISTORS

Introduction
In 1956 the Nobel prize for physics was awarded to Shockley, Bardeen and Brattain for the invention of the transistor. The transistor has enabled the modern telecommunications revolution.


The Transistor
The transistor is an electronic device that transforms small electrical currents (and voltages) into larger copies of the original - it is what is called an amplifier and is said to have 'gain' (magnification). The transistor has three wire connections called; the emitter (E), the base (B) and the collector (C). By wiring the device up with other simple components an amplifier can easily be constructed. A typical transistor has a gain of about 100 times.
The physical theory describing the transistor involves understanding the movement of electrons (and the absence of electrons - holes) in P and / or N doped semiconductor materials. What follows here is not a detailed account of the theory but a simple set of experiments that demonstrates the transistor working.

How it works
A diode is a two wire electronic component that only conducts electricity when connected the 'correct way round' i.e. with the potentials applied correctly. It is composed of a P and N semiconductor junction. The transistor is a three wire component composed of a sandwich of either PNP or NPN junctions.

Electrically it is as if the transistor is composed of two diodes wired back to back. The common middle region (the base - B) of the transistor is much thinner than the other two regions.
 Because the diodes are opposed to each other no current would normally flow when a voltage is applied between the emitter and the collector - EC (although there may be a tiny leakage current).
If a voltage is applied across BE (B positive and E negative for an NPN transistor) this junction will be forward biased so a current will flow in this circuit. However, because the base region is very thin (and also because when wired up correctly the collector is at a high potential and so attracts electrons) as much as 99% of this current will actually flow right across the base region to reach the collector (C). So we have actually made the EC circuit of the transistor conduct by applying a current into B (set up by a small voltage across BE).
Now the current flowing from the emitter must be equal to the sum of i) the 99% arriving at the
collector and ii) the 1% that is left flowing through the base. So the base current is small, only 1%
or so. But as we have seen the collector current can not exist without the little base current and so
it is effectively controlling the collector current. This collector current is a larger copy of the base
signal and so we find the transistor produces a current gain! Current gains of 100-200 are typical
for a transistor. Usually the EC part of the circuit is used as the output and the base is used as the
input of the amplifier.
The EB circuit is low voltage and low current while the EC is at a much higher potential and higher
current. As power = voltage x current we must therefore have a higher power in EC and so a
power-gain is possible with such a simple circuit. Of course the transistor does not amplify this
small base signal by 'magic', the extra power is derived from the supply driving the transistor
circuit. The transistor needs a battery, or other supply, to work its 'magic'.