BJT
full spelling ： Bipolar Junction Transistor
Bipolar junction transistor or BJT is a type of transistor which consists of three regions of doped semiconductors. Bipolar Junction Transistor consists of a pair of PN Junction Diodes that are joined back-to-back where one kind of semiconductor is placed in between two others.
Bipolar transistors are classified as either NPN or PNP according to the arrangement of their N-type and P-type materials. An NPN transistor is formed by introducing a thin region of P-type material between two regions of N-type material. A PNP transistor is formed by introducing a thin region of N-type material between two regions of P-type material.