The TO Package Family Explained — The Dimension Code from TO5 to TO56

I. Where Does the TO Package Come From? Decoding the Naming Rules

The TO (Transistor Outline) package was born in the transistor era — a unified dimensional standard JEDEC defined for transistor housings, later inherited by optoelectronic devices and still going strong after more than sixty years.

The number after TO is only a “designation”: TO5, TO8, TO9, TO18, TO39, TO46, TO56… A bigger number does not mean a bigger size; it only reflects the order in which they were defined. Each designation corresponds to a fixed base diameter, pin count, and pin layout, and is universal worldwide — a TO56 header from supplier A also fits at supplier B.

The interchangeability that standardization brings is the foundation of the TO package’s enduring vitality.

II. How to Choose Among Common Models? One Comparison Tells All

TO packages commonly used in optoelectronics, classified by base diameter and typical application:

TO5/TO18— base diameter about 9.2 mm, 3–4 pins; small detectors and sensors.

TO39— diameter about 9.1 mm, up to 8 pins; the classic model when more pins are needed.

TO46— flat seating plane favors die mounting; commonly used for laser diodes.

TO56— diameter about 5.6 mm, compact; the mainstream choice for fiber-communication lasers.

TO9— larger base, suited to high-power or multi-pin scenarios.

Three things to check when selecting: the chip size (is the mounting surface large enough), the pin count (how many signal paths the circuit needs), and the power (heat-dissipation demand). Xuri’s TO header series covers the whole family — TO5, TO9, TO18, TO39, TO46, TO56 — and is supplied as complete kits with optical windows and caps.

[Next Episode] Next time: the “standard fit” of high-end optical modules — a deep dive into butterfly packages.

[Glossary]

1. TO Package (Transistor Outline Package): a cylindrical metal hermetic package derived from the transistor-housing standard

2. JEDEC (JEDEC Solid State Technology Association): the international body that develops standards for semiconductor devices