TO-126

TO-126 is a type of semiconductor package for devices with three pins, such as transistors.

[1] The package is rectangular with a hole in the middle to allow for easy mounting to a board or a heat sink.

On one side of the package typically a metal sheet is exposed, with the transistor die bonded to the other side of the metal sheet inside the package.

[2] This allows for an efficient heat transfer from the transistor die to an external heat sink but also implies that the metal sheet is electrically connected to the die (for a bipolar junction transistor usually the collector is connected to this metal sheet).

[3] In the updated JEDEC outline system, the package is numbered as TO-225AA.