Bipolar junction transistor

A bipolar junction transistor (BJT) is like a tiny switch. It can be used to control the flow of electricity through different circuits. A BJT has three connections (called terminals), called the emitter, collector and base. When the base is operated with a small electric current, it can open the path between the emitter and collector, allowing electricity to flow through the circuit. This flow of electricity through the BJT is controlled by the base. Think of it as a switch that you can turn on and off using a small current.