To-18 Package
The TO-18 package is a small JEDEC-registered round metal can, roughly 5 millimetres across, used for small-signal transistors and diodes, and it is the outline in which the 2N2222 first became famous. It shares its construction with larger cans such as the TO-5: a flat metal header, glass-sealed wire leads and a welded cap. Its identity is size. The roughly five-millimetre body is about half the width of a TO-5, with three thin leads on a small circle, a tab on the flange to show the emitter, and a height of a few millimetres, which made it the standard home of general-purpose bipolar transistors of the 1960s.
The TO-18 is the small-signal member of the metal-can family, where the part number tells the story. A 2N2222 or 2N2222A in TO-18 is rated for a collector current of the order of 800 milliamperes, a collector-emitter breakdown voltage of 30 to 40 volts, a power dissipation of about half a watt, and a transition frequency of a few hundred megahertz; these numbers are exactly the middle of the range where a small can is enough. The case is small, so heat leaves through the leads and a thin air layer rather than a heat sink, and a device pushed beyond its rating simply gets hot. This is why the can is used for amplifiers, oscillators, relay drivers and switching rather than for power stages.
The 2N2222 is the standard example, and its history shows how outlines and part numbers separate. The 2N2222 was a widely second-sourced NPN silicon switch and amplifier of the early 1960s, available in TO-18 metal; the 2N2218 and 2N2219 versions with similar silicon were sold in the larger TO-5. Because of this, the part number does not give the can, and an engineer buying a replacement must check the outline code. Later the same die was moulded as the PN2222 in a plastic TO-92 and as the MMBT2222 in a tiny surface-mount SOT-23, so the electrical identity survived while the housing changed twice. The TO-18 version remains available for rugged or military use.
The small can had real engineering advantages for high-frequency work. A smaller header means shorter leads inside the package and less stray capacitance between them, so the transistor's own frequency limit is less degraded by its housing. Four-lead variants, in the style of the TO-72, bring a fourth connection out to the case so it can be grounded as a shield, which helps radio-frequency amplifiers and low-noise stages. Metal also blocks light, which matters because a junction in clear glass behaves as a weak photodetector. The compact can additionally allowed several devices to be mounted densely on early circuit boards, in an era before surface-mount parts.
It was displaced by plastic for cost reasons, and several pitfalls follow from its metal construction. A moulded TO-92 transistor costs a fraction as much and can be inserted by machine, so by the 1970s plastic covered nearly all consumer circuits, and the TO-18 persisted in aerospace and test equipment. Many users forget that the metal case of a TO-18 often connects to the collector internally, so a case touching a chassis or another conductor creates a short. Another frequent mistake is treating pin order as universal: datasheets draw metal-can pinouts from the bottom view, plastic ones from the front, and even same-chip variants in different housings can order emitter, base and collector differently, so a transistor mounted by habit can end up reversed and overheated.
Read the TO-18 package as the 5-millimetre metal can of the small-signal era: a size class rather than a part type, best remembered as the outline in which the 2N2222 began, before the same silicon moved into plastic.