The TIP122(RM0387) is an NPN Epitaxial Darlington Transistor. Mainly used for Medium Power Linear Switching Applications and Complements to TIP125/126/127.
In electronic, the Darlington transistor (often called a Darlington pair) is a compound structure consisting of two bipolar transistor (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. This configuration gives a much higher common/emitter current gain 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.