Transistors control the amount of current in a circuit. Generally, transistor packaging determines the transistor's frequency characteristics and power rating. Power transistors generally come with larger packages to enhance cooling. The mode of use also determines the transistor package. Switching transistors, for example, dissipate a lot of heat thereby requiring them to have a much larger package of certain materials. Transistor packages come in many types, and the number is growing with increased use.
TO-3 is a package similar to a metal can that is for transistors with high power dissipation ratings. This type of transistor package is useful for power transistors and other semiconductor devices in order to switch high-power circuits. The package is entirely metal and designed to hold large heat sinks. It also comes with a sealing that provides protection against environmental factors.
TO-5 is a metal package for transistors with low leads, such as op-amp -- operational amplifier -- transistors. TO-5 allows manufacturers to mount the microchip on the die pad, which is then welded with a high current. Manufacturers also use this type of transistor package on integrated circuits and other semiconductor devices for the same purpose.
TO-8 is a metal package for integrated circuits and special transistors in resistor networks and transducers. Manufacturers generally prefer this type of transistor package for any semiconductor device that has pin counts ranging from two to 16.
TO-46 is a metal package primarily designed for three-terminal semiconductor devices. This type of transistor package provides protection against moisture and contaminants, and manufacturers use it on low lead circuit transistors where the manufacturers don't expect much heat dissipation.
TO-66 is a package for high-power dissipation transistors like silicon controlled rectifiers and other switching semiconductor devices. A large package base allows manufacturers to attach the transistor to a large heat sink in order to increase heat dissipation. This type of transistor package is similar to TO-3 but slightly smaller.
Manufacturers use TO-72 in three-terminal devices like bipolar and field-effect transistors. Also called a metal header, the TO-72 package provides protection against moisture and other environmental contaminants.
TO-126 is a plastic transistor package that manufacturers use for high-power transistors. This type of transistor package allows manufacturers to attach large heat sinks to it. Manufacturers use TO-126 in discrete devices like thyristors and other switching transistors.
TO-218 is a type of transistor package that manufacturers use on low lead count transistors, thyristors and other devices. This type of transistor package provides high power dissipation when manufacturers connect it to a large heat sink.
TO-258 is a metal package that also includes glass and ceramic feedthroughs for insulating the transistor leads. You can usually find TO-258 in thyristors, integrated circuits and mission-critical transistors that are expected to provide high reliability.
TO-264 is a plastic transistor package for high power transistors, thyristors and other semiconductor devices used in high power circuits. Manufacturers use this type of transistor package on semiconductor devices that have a lead count ranging from two to five.