TO-3

TO stands for "Transistor Outline" and relates to a series of technical drawings produced by JEDEC.

[1] The TO-3 case has a flat surface which can be attached to a heatsink, normally via a thermally conductive but electrically insulating washer.

[2][3] The lead spacing was originally intended to allow plugging the device into a then-common tube socket.

Insulating washers may be made of mica or other materials with good thermal conductivity.

The case is used with high-power and high-current devices, on the order of a few tens of amperes current and up to a hundred watts of heat dissipation.

The metal-to-metal and metal-to-glass joints provide hermetic seals that protect the semiconductor from liquids and gases.

[7]: 215 TO-204 is intended to replace previous definitions of flange-mounted packages with a 10.92 mm (0.430 in) pin spacing.

Size comparison of BJT transistor packages, from left to right: SOT-23 , TO-92 , TO-126 , TO-3
3D model of TO-3 package
Typical TO-3 mounting profile, with insulator from chassis
Inside of an MJ1000 Darlington transistor in TO-3 package
Dimensions TO-3 package in mm [ 1 ]
Power amplifier integrated circuit ( CEMI UL1403) in a TO-3 package variant
AD133 transistor in a TO-41 package. The third pin connected to the case