Darlington transistor

In electronics, the Darlington transistor (often called a Darlington Pair) is a compound structure consisting of two bipolar transistors (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one.[1] This configuration gives a much higher current gain (written β, hfe, or hFE) than each transistor taken separately and, in the case of integrated devices, can take less space than two individual transistors because they can use a shared collector. Integrated Darlington pairs come packaged singly in transistor-like packages or as an array of devices (usually eight) in an integrated circuit.

=Reference=
 * http://en.wikipedia.org/wiki/Darlington_transistor