A Bipolar Transistor or BJT (Bipolar Junction Transistor) is a solid state, three-pin device made from three layers of silicon. A BJT is designed to amplify current, bipolar transistors can also function as a switch. There are two main types of transistor, PNP (positive negative positive) or NPN (negative positive negative).How a Bipolar transistor is madeA bipolar transistor is made by joining two signal diodes back to back creating two PN junctions connected in series, sharing a common P or N terminal. By nature, silicon does not normally conduct electricity well. However, when silicon is treated with certain chemicals or impurities we can make the material and electrons behave in a different way. This process is called doping. The doping process improves the semiconductors ability to conduct electricity.What does a bipolar transistor do?As we said before a bipolar transistor has two possible functions, switching, and amplification. Due to the devices three layers of doped semiconductor material, supplying the transistor with a signal voltage enables the discrete component to act as an insulator or a conductor. This clever change provides the transistors with two basic functions, switching (digital) electronics or amplification (analog) electronics.Types of transistorTransistors are available in panel, surface and through hole mounting options in plastic package or metal can versions. All have three terminals or pins, the Base, the Collector, and the Emitter.Where are bipolar transistors used?Transistors are one of the most widely used discrete components in electronic designs and circuits. Transistors are used for the amplification of all types of electrical signals in circuits made up of individual components rather than ICs (integrated circuits).