Principal of Bipolar junction transistor:

A bipolar junction transistor (BJT) is a three-terminal electronic device that constructed of doped semiconductor material and might be employed in amplifying or switching applications. Bipolar transistors are so named as their operation includes both electrons and holes. Charge flow in a BJT is because of bidirectional diffusion of charge carriers beyond a junction among the two regions of different charge concentrations. This type of operation is contrasted with unipolar transistors, like field-effect transistors, in which only one carrier type is included in charge flow because of drift. By design, most of the BJT or bipolar junction transistor collector current is because of the flow of charges injected from a high-concentration emitter into the base in which they are minority carriers that diffuse toward the collector, and so BJTs are categorized as minority-carrier devices.

Schematic symbols for PNP- and NPN-type BJTs.