A bipolar junction transistor is a type of transistor that relies on the contact of two types of semiconductor for its operation.
BJTs can be used as amplifiers, switches, or in oscillators. BJTs can be found either as individual discrete components, or in large numbers as parts of integrated circuits.
Bipolar transistors are so named because their operation involves both electrons and holes.
These two kinds of charge carriers are characteristic of the two kinds of doped semiconductor material; electrons are majority charge carriers in n-type semiconductors, whereas holes are majority charge carriers in p-type semiconductors.