A Bipolar Junction Transistor (BJT) is a type of transistor. It is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. That is, charge flow is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. This mode of operation is in contrast with unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due the flow of charges injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the