The first bipolar junction transistor was invented in 1947 at Bell laboratories. “Two polarities” is abbreviated as bipolar, hence the name Bipolar junction transistor. BJT is a three terminal device with Collector (C), Base (B) and Emitter (E). Identifying the terminals of a transistor requires the pin diagram of a particular BJT part, it will be available in the datasheet. There are two types of BJT - NPN and PNP transistors. In this tutorial we will talk about the NPN transistors. Let us consider thetwo examples of NPN transistors - BC547A and PN2222A, shown in the images above.