To-5 Package
The TO-5 package is a JEDEC-registered round metal can about 9 millimetres across, with a flange at its base and three or more thin leads projecting from the bottom, used for medium-power transistors and for many early integrated circuits. TO stands for transistor outline, the numbering system the standards body used to catalogue the shapes in which transistors were sold. As a mechanical standard the TO-5 matters because a circuit designer could pick any device in that outline and know it would fit the same socket, the same heat-sink clip and the same drawing, whoever made it.
The TO-5 is the larger sibling of a family of round cans, and it was the workhorse for devices that needed more than a small-signal case. Its cap is roughly 9 millimetres in diameter and a few millimetres tall, and the three leads of a transistor emerge from the base on a circle, with a small tab on the flange marking the emitter side so the leads can be identified from below. Typical devices rated about half a watt to a watt in free air, and several watts when the case is attached to a heat sink, so it suited audio drivers, switches and radio-frequency stages. The closely related TO-39 has a lower cap and is often confused with it; the two share the same footprint, which is why datasheets and distributors mix the names.
Several landmark silicon transistors were sold in TO-5 style cans, and the exact outline of each is a place where memory often fails. Fairchild's diffused-base mesa NPN transistor 2N697, an early high-volume silicon device from around 1958 to 1960, was supplied in a TO-5 can, and so was the planar 2N1613 that followed in 1960. The 2N2218 and 2N2219 family, designed as higher-current relatives of the 2N2222, also used it. The 2N2222 itself, by contrast, was originally a TO-18 part, the smaller can, and only later appeared in plastic and surface-mount forms; the shared numbers lead many people to assume it is a TO-5. Checking the datasheet for the outline code, instead of assuming it from the part number, is the reliable habit.
The TO-5 style also housed the first widely used integrated circuits, because a round can with extra leads was a ready-made hermetic package. Early operational amplifiers such as Fairchild's 709 and later the 741 were sold in an eight-lead metal can in the TO-5 style, with the leads spaced evenly around the circle and a tab marking the first pin, often registered as TO-99. Using the existing can meant that the chip, wire bonds and glass seals were solved problems, and the same round outline supported analogue ICs for years even as dual in-line packages arrived. Because the leads are round and spaced on a circle, circuits were mounted in sockets or bent to fit, which looks awkward next to a flat board layout.
The reasons the TO-5 faded are cost and board layout, not electrical performance. A welded metal can with glass seals costs more than a moulded plastic body and cannot be handled by the pick-and-place machines that populate modern boards. As a result, plastic outlines such as TO-92 and DIP packages replaced it in consumer and industrial work, and surface-mount packages followed. TO-5 devices continue to be made in military and aerospace qualification flows, where a hermetic can and wide temperature range remain valuable, and an entire ecosystem of replacement parts exists for old equipment. A common pitfall is assuming the case is electrically neutral: in many TO-5 parts the metal can is connected internally to the collector or another terminal, so touching it to a chassis or another pin can short the circuit.
Read the TO-5 package as a standard-size round metal can that carried the transistor era's medium-power devices and the first analogue integrated circuits: a mechanical convention, not a device type, whose outline tells you the size and ratings class but never the part's identity.