Bipolar Junction Transistors (BJTs)

A bipolar junction transistor (BJT) is a three-terminal active electronic device that has a capability to amplify current. They are constructed of doped semiconductor materials. The three terminals are commonly known as Base, Emitter and Collector. The invention of BJT was a revolution in electronic industry. Before its invention, large size vacuum tubes were used for amplification which were less efficient, mechanically fragile and high power consuming. With the use of BJTs, the electronic design and circuits became light weight, inexpensive, and more reliable. It is called bipolar because the current flow is mainly due to two types of charge carriers: electrons and holes.
If you are a beginner student of electronics or a hobbyist, it is very important to get familiarized with the basic operating principles of BJTs, because it is the basic building block of all higher electronics technologies. I am going to provide the links below that will give you much deeper understanding of how BJT works.

• Theory of Bipolar Junction Transistors : Check this link to learn the physics inside a BJT, BJT types, symbols, and current flow directions.
• BJT Configurations and Biasing: A BJT is an active component, so it requires external power source. In order to operate properly, appropriate voltages should be applied to the three terminals of a BJT, which is often called biasing a BJT. Further, a BJT can be configured in different ways depending upon types of applications.
• BJT as a Switch: Find out how a BJT can be used as a switch.
• Practical Transistor Circuits 1  Practical Transistor Circuits 2
• Transistor from Wikipedia