Bipolar junction transistor

A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the contact of two types of semiconductor in order for it to work. BJTs can be used as amplifiers, switches, or in oscillators. BJTs can be found either on their own, or in large numbers as parts of integrated circuits.
They are named "bipolar" transistors because their operation involves both electrons and holes.
The current gained is measured in hfe, Forward Current Gain. Typical amount can be between 200-350.