thumb|Size comparison of Bipolar junction transistor|BJT transistor packages, from left to right: [[SOT-23, TO-92, TO-126, TO-3]] thumb|3D model of TO-3 package In electronics, TO-3 is a designation for a standardized metal semiconductor package used for power semiconductors, including transistors, silicon controlled rectifiers, and, integrated circuits. TO stands for "Transistor Outline" and relates to a series of technical drawings produced by JEDEC.
thumb|Size comparison of Bipolar junction transistor|BJT transistor packages, from left to right: [[SOT-23, TO-92, TO-126, TO-3]] thumb|3D model of TO-3 package In electronics, TO-3 is a designation for a standardized metal semiconductor package used for power semiconductors, including transistors, silicon controlled rectifiers, and, integrated circuits. TO stands for "Transistor Outline" and relates to a series of technical drawings produced by JEDEC.
The TO-3 case has a flat surface which can be attached to a heatsink, normally via a thermally conductive but electrically insulating washer. The design originated at Motorola around 1955 from a group headed by Dr. Virgil E. Bottom. who was director of research of the Motorola Semiconductor Division. The first use of this design was for the germanium alloy-junction power transistor 2N176 – the first power transistor to be put into quantity production. The lead spacing was originally intended to allow plugging the device into a then-common tube socket.
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