A Darlington transistor is a type of transistor that combines an NPN and a PNP transistor into a single package. It is used to amplify current or voltage, as it is able to provide a much higher current gain than a single transistor. It is useful in applications such as power supply circuits, audio amplifiers, and motor controls. As the voltage across the Darlington transistor increases, current flows through the base-emitter junction of the NPN transistor and into the base of the PNP transistor. This in turn generates a large current gain, allowing the Darlington transistor to be used as an amplifier of both voltage and current.