TO-18

The name is from JEDEC, signifying Transistor Outline Package, Case Style 18.

For example, diode lasers such as those found in CD players may be packaged in TO-18 cases with a lens.

[4] The fourth wire is typically connected to the metal case as a means of electromagnetic shielding for radio frequency applications.

TO-206 is intended to replace previous definitions of packages with leads arranged in a circle with a diameter of 2.54 mm (0.100 in).

A new package with 3 leads and a cap height of 1.40 mm (0.055 in) (i.e. smaller than TO-46) is added as TO-206-AD.

BC108 family transistors from various manufacturers in a TO-18 package.
Cross section of 2N2222 transistor in metal TO-18 package, showing connection wires between external pins and die .
Size comparison between (left to right) TO-39 , TO-18, and TO-92 packages.
Phototransistor in a modified TO-18 package with lens on top.