BIPOLAR JUNCTION TRANSISTOR (BJT)

The transistor is a three terminal semiconductor device which is used for multiple applications such as amplifier, switch etc...

Bipolar Junction transistor or BJT uses two charge carriers for it's operation holes and electrons. The three terminals of BJT are emitter, base and collector. The emitter is heavily doped semiconductor and moderately sized, the base is lightly doped semiconductor and is smaller in size, the collector is moderately doped and is larger in size among the three terminals. There are two types of BJT based on the semiconductor material used for the three regions. They are
1) NPN
2) PNP

NPN is the most commonly used BJT.  the base is p-type semiconductor while the emitter and collector are n-type semiconductor materials. The P type semiconductor is sandwiched or placed between two n-type semiconductors. The BJT is used in three basic configurations they are

1)Common base where base is common between emitter and collector. This is used as buffer.
2) Common emitter where emitter is common between input and output , this configuration is most commonly used and have universal applications such as amplification and other important circuits.
3) Common collector where collector is common between input and output.

The charge carriers usually flow from emitter to collector and the current direction is indicated by the arrow Head at the emitter end. In PNP transistor holes are the majority carriers while electrons are minority carriers where as in NPN holes are minority carriers and electrons are majority carriers.