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A bipolar junction transistor (BJT) is a current-driven semiconductor element that uses a small amount of current to control the flow of a larger electric current. In contrast to a unipolar transistors, such as a field-effect transistors which uses only one kind of charge carrier, a BJT uses both electrons and electron holes as charge carriers. BJT Transistors can be used to amplify weak electrical signals and are also used as oscillators or as switches.