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Transistors - I think it very remarkable how it suddenly appeared on the scene, and how in a relatively short space of time developed into the devices we have today.

The word transistor is made up from two words – transfer and resistor.

First and foremost from outward appearances it has three legs, and these three legs have names, their names are; Collector, Base and Emitter.

A small current in the base controls a large current in the collector this gives us a device that can amplify signals, or can switch larger currents by the application of a small current in the base.

GAIN can be found by dividing the collector current by the base current and can be anything from a figure of 30 to 800

Does it help to know about the structure of matter and the in depth theory of semiconductor materials when using transistors in practice? Probably not, but it helps to know there are two types of transistor

The difference between the two transistor types is the way in which we connect the power supply. The one transistor type is complimentary to the other, and this is very often put to good use in electronics.

Usually if you take the explanation for a NPN transistor you can apply it in exactly the same fashion for a PNP transistor, just reverse the polarity of the power supply connections and reverse the polarity of the capacitors.

A transistor is made up of two types of silicon: N type and P type.

If you test various materials for how they conduct electricity then copper and aluminium would be classed as very good conductors indeed.

If you take a length of pure silicon and did the same test it would be a very bad conductor of electricity.

The silicon is made into a SEMICONDUCTOR by introducing impurities such as Boron or Arsenic. This is called doping.

Silicon doped with Boron produces a P type semiconductor.

Silicon doped with Arsenic produces an N type semiconductor.

Note: Doping silicon with Gallium Arsenide produces the startling effect of turning electricity into light, and is used in the manufacture of light emitting diodes (L.E.D.s.)

P type semiconductor has a deficit of electrons called holes in its composition.

N type semiconductor has a surplus of electrons in its composition.

Now if a piece of P type silicon is sandwiched between two N type pieces of silicon we have the construction of an NPN transistor.

Conversely a piece of N type silicon is sandwiched between two P type pieces of silicon we have the construction of a PNP transistor.

Note the base is an extremely thin piece of P type silicon (0.01mm) in comparison to the size of the silicon used for the collector and emitter in a transistor.

The Depletion Layer

A strange thing happens at the base emitter junction when N and P type semiconductors are put together such as the P type of the base and the N type of the emitter some of the holes migrate across the junction from the base into the emitter and some of the electrons from the emitter migrate across from the emitter into the base creating a depletion layer. This means that for a silicon transistor a voltage of approximately +0.6 volts has to be applied between the base and emitter before any conduction between the collector and emitter will occur.

Practically this means the transistor has to be biased correctly in order for it to operate.

This is achieved with resistors as shown below in its simplest form: 

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