Transistor Construction

At the heart of a transistor is a diode junction, which consists of a piece of n-type semiconductor and a piece of p-type semiconductor.



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We have here a p-n junction.


A bipolar transistor consists of two p-n junctions back to back like this:


The arrows represent conventional current flowing from positive to negative. Semiconductors are doped so that some have an excess of electrons, n-type material, while others have a deficiency of electrons called holes.  These are found in p-type materials.


Transistors conduct when the base-emitter voltage is about 0.7 V.  Transistors are current controlled devices.  The bigger the base emitter current, the bigger the collector emitter current.

The transistor can be used as an amplifier or a solid state switch.



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