A bipolar transistor (BT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor such as a field effect transistor uses only one kind of charge carrier. A bipolar transistor allows a small current to be applied to one of its pins to control much more current flowing between the other two pins, making the device capable of amplification or switching.

BTs use two junctions between two types of semiconductors, n-type and p-type, which are regions in a single crystal of the material. The transitions can be performed in several different ways, such as changing the doping of the semiconductor material as it grows, depositing metal pellets to form alloy transitions, or by methods such as diffusion of n-type and p-type dopants into the crystal. The superior predictability and performance of junction transistors soon supplanted the original point transistor. Diffused transistors, along with other components, are integrated circuit elements for analog and digital functions. Hundreds of bipolar junction transistors can be fabricated in a single circuit at a very low cost.

Bipolar transistor integrated circuits were the main active devices of the generation of mainframes and minicomputers, but most computer systems now use integrated circuits based on field effect transistors. Bipolar transistors are still used today for signal amplification, switching, and digital circuits. Specialized types are used for high voltage switches, RF amplifiers, or for switching high currents.