Bipolar junction transistor:

Transistors are so named as they conduct via using both majority and minority carriers. The bipolar junction transistor that is abbreviated as BJT, the first type of transistor to be mass-produced, is a mixture of two junction diodes, and is made up of either a thin layer of p-type semiconductor sandwiched among the two n-type semiconductors (an n-p-n transistor), or a thin layer of n-type semiconductor sandwiched among the two p-type semiconductors (a p-n-p transistor). This construction generates two p-n junctions: a base-emitter junction and a base-collector junction, separated via a thin region of semiconductor termed as the base region (two junction diodes wired together with no sharing an intervening semiconducting region will not make a transistor).

The BJT or bipolar junction transistor has three terminals that are corresponding to the three layers of semiconductor - an emitter, base, and collector. It is helpful in amplifiers as the currents at the emitter and collector are controllable through a comparatively small base current." In the active region, the emitter-base junction is forward biased and electrons are injected into the base region, in an NPN transistor operating. Since the base is narrow, most of these electrons will diffuse into the reverse-biased (electrons and holes are made at, and move away from the junction) base-collector junction and be swept into the collector; may be one-hundredth of the electrons will recombine in the base that is the dominant mechanism in the base current. Through controlling the number of electrons which can leave the base, the number (digit) of electrons entering the collector can be controlled. Collector current is almost β (common-emitter current gain) times the base current. It is commonly greater than 100 for small-signal transistors but can be much smaller in transistors intended for high-power applications.