This document provides an overview of bipolar junction transistors (BJT). It discusses the basic construction of BJTs, including that they have two PN junctions and three terminals - emitter, base, and collector. The document explains that BJTs can be either NPN or PNP, depending on whether an N-type or P-type material is sandwiched between two other materials. It then describes the roles of the emitter, base, and collector terminals and shows symbols for NPN and PNP BJTs. The document is meant as an introduction to BJTs and lists several topics that will be covered related to BJT configurations, characteristics, regions of operation, biasing and ampl