TO Packages: TO-92 & TO-220 Transistor Outline Explained

TO Packages (Transistor Outline)

TO packages (Transistor Outline) are the most common package types for discrete semiconductor components such as transistors and certain integrated circuits (ICs). The various package types differ mainly in their size and their ability to handle heat dissipation (power loss). Below is a brief description of the two package types most frequently used in experiments…

The TO-92 Package

Close-up of a TO-92 semiconductor package. This frequently used plastic case serves to house small-signal transistors and simple integrated circuits. — A TO-92 Package.

The TO-92 is the smallest and most cost-effective package type for semiconductors…

Application	Small-Signal Semiconductor Components (e.g., for switching and amplifying small currents).
Material	Made entirely of plastic (epoxy resin).
Heat Dissipation	Very low. Due to the lack of a heatsink, the TO-92 can only dissipate very little power loss. It is susceptible to overheating at higher currents.
Note for Experiments	The leads do not fit directly onto standard breadboards. You must carefully bend the pins into the correct shape with pliers.
Examples	Transistors of the BC547, BC557 series (NPN/PNP).

The TO-220 Package

Close-up of a TO-220 semiconductor package. This larger package, featuring the characteristic metal tab, is primarily used for power components that require active cooling. — A TO-220 Package

The TO-220 is a more robust package and is the industry standard for medium and higher power electronics.

Application	Power Components such as power transistors (MOSFETs, BJT), thyristors, and voltage regulators with higher current requirements.
Material	Plastic (epoxy resin) and features a characteristic metal tab.
Heat Dissipation	Very good. The metal tab allows for direct mounting to a heatsink. This enables the TO-220 package to dissipate significantly higher power losses (several Watts) than the TO-92.
Note for Experiments	The leads fit directly onto standard breadboards.
Examples	Linear voltage regulators of the 78xx series (e.g., 7805, 7809).