TO-204 Transistor Package

TO-204 Metal Can, Flange Mount

The TO-204 is a metal through-hole device, similar to a TO-23 Case package, or TO-3 Case package [and a number of others]. The drawing above shows the normal pinout for the TO-204 package when used to hold a FET, note that the metal case is the Drain connection. The photograph below shows the normal TO-204 mounting method.

Case mount. [Flange Mount]. A type of package which provides a method of attaching one surface of the semiconductor device to a heat sink or metal surface to achieve thermal management of the case temperature. The case of this device is connected to the Drain [Collector]. The mounting method is also called Chassis Mount or Panel Mount. Case temperature. Case temperature is that temperature measured at a specified point on the case of a semiconductor device.

TO-3 Metal Case

Normal TO-204 heatsinks are a Black Anodized Aluminum Alloy with 2 mounting holes. However many times the TO-204 is attached directly to the chassis and the metal panel of the case is used as a heat-sink. Many times a TO-204 is mounted on the outside of a chassis so that it uses the metal case as a heat sink and the cooler air external to the chassis as a cooling source.
Note the insulator used between the TO-3 package and the metal panel it's attached to in the photograph. The plastic insulator happens to have the same diamond shape as the base of the TO-3 package, but of course has holes or cut-outs located where the leads of the device would be.
Companies producing Heat Sink, Manufacturers & vendors.

Example devices using the TO-204 package;
NPN Darlington Power Transistor 2N6283 and 2N6284
PNP Power Transistor 2N6437 and 2N6438
NPN Power Transistor 2N6338 and 2N6341
N-Channel Field Effect Transistor 2N6756, 2N6758, 2N6760, 2N6762
TO-204AE
N-Channel Repetitive Avalanche Field Effect Transistor [FET] 2N6764 and 2N6766
TO-204AA
N-Channel Repetitive Avalanche Field Effect Transistor [FET] 2N6768 and 2N6770
N-Channel Field Effect Transistor [FET] 2N6902, and 2N6904
N-Channel Radiation Hardened Field Effect Transistor 2N7291, 2N7293, 2N7295 & 2N7297