Transistor Outline (TO) Package

A transistor is a three-terminal device cleverly encapsulated in plastic or metal housing, with three leads protruding. This type of packaging is commonly referred to as “TO packaging,” where “TO” stands for “Transistor Outline.” It consists of two parts: the TO header and TO cap. The purpose is to provide physical and environmental protection for the transistor, facilitating easy integration into circuit boards and enhancing heat dissipation to ensure the stability and reliability of the transistor.

Transistor Outline Package

TO Header

The TO header refers to the part that connects the leads, with each of the three leads corresponding to the transistor’s three critical electrodes. Depending on space and chip size, different TO headers can be chosen, such as a housed TO header or a stamped TO header. Essentially, this header functions similarly to a socket plug, inserted into designated holes on the printed circuit board during use.

Transistor Outline header

TO Cap

The TO cap is like a hat covering the top of the transistor package. This compact design protects the transistor from external environmental factors and provides additional mechanical support for the encapsulation. TO caps come in various types, including:

Spherical Lens Cap: Specifically designed for laser and fiber optic coupling applications, aiming to achieve collimation and point-to-point imaging.

Ball lens cap

Mini Lens Cap: Boasting high reliability and security, particularly suitable for sensor projects.

Mini lens cap

Types of TO Packaging

NamePicture
TO-3TO-3 package
TO-5TO-5 package
TO-8TO-8 package
TO-18TO-18 package
TO-36TO-36 package
TO-39TO-39 package
TO-46TO-46 package
TO-52TO-52 package
TO-66TO-66 package
TO-72TO-72 package
TO-92TO-92 package
TO-126TO-126 package
TO-202TO-202 package
TO-218TO-218 package
TO-220TO-220 package
TO-226TO-226 package
TO-254TO-254 package
TO-257TO-257 package
TO-258TO-258 package
TO-259TO-259 package
TO-264TO-264 package
TO-267TO-267 package

The mentioned TO transistor packaging types are common, and with increasing demand, more types may emerge in the future. Nevertheless, the ultimate goal of these packages is similar, differing mainly in their emphasis. For example, TO-3 is much larger than TO-46 and is primarily used for high-power applications, whereas TO-46 is utilized in VCSELs, sensors, and similar applications. Additionally, some packages exist in variant forms, serving as upgrades or improvements. Examples include TO-3 and TO-66, TO-5 and TO-39, TO-220 and TO-263, and so on.

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