Transistors are semiconductor devices that can amplify or switch electronic signals and power. They operate as either switches or amplifiers and are made of doped silicon. There are two main types - bipolar junction transistors (BJT) and field-effect transistors (FET). BJTs use both electron and hole charge carriers and have three terminals - base, collector, and emitter. Current flowing into the base terminal controls the much larger currents in the collector and emitter. This amplification property allows BJTs to be used as amplifiers or electronic switches in many devices.