A bipolar junction transistor (bipolar transistor or BJT) is a type of transistor that uses both electrons and holes as charge carriers.
Unipolar transistors, such as field-effect transistors, use only one kind of charge carrier.BJTs use two junctions between two semiconductor types, n-type and p-type.
BJTs are manufactured in two types: NPN and PNP, and are available as individual components, or fabricated in integrated circuits, often in large numbers.
BJTs can be used as amplifiers or switches. This ability gives them many applications in electronic equipment such as computers, televisions, mobile phones, audio amplifiers, industrial control, and radio transmitters.