 A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material.The junctions can be made in several different ways, such as changing the Doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n -type and p-type doping substances into the crystal.