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Back side of a power supply box with an MJ3001 Darlington transistor in a TO-3 package (right side).

In electronics, TO-3 is a designation for a standardized metal semiconductor package used for transistors and some integrated circuits. The TO element stands for "transistor outline" and refers to a series of technical drawings produced by JEDEC.

The TO-3 metal can package is commonly used for power transistors, silicon-controlled rectifiers, and, occasionally, integrated circuits. The semiconductor die component is mounted on a metal plate with a metal can crimped on top of it, providing good heat conduction and durability. TO-3 packages usually have two leads, the case being the third connection, though devices with four leads exist and more leads are possible. The leads pass through the metal base plate and are sealed with glass. The metal case is connected to the internal device. The TO-3 case has two mounting holes. The design originated at Motorola around 1955. The lead spacing was originally intended to allow plugging the device into a then-common tube socket.[1]

Typical applications[edit]

The metal package can be attached to a heat sink, making it suitable for devices dissipating several watts of heat. Thermal compound is used to improve heat transfer between the device case and the heat sink. Since the device case is one of the electrical connections, an insulator may be required to electrically isolate the component from the heatsink. Insulating washers may be made of mica or other materials with good thermal conductivity.

The case is used with high-power and high-current devices, on the order of a few tens of amperes current and up to a hundred watts or so heat dissipation. The case surfaces are metal for good heat conductivity and durability. The metal-to-metal and metal-glass joints provide hermetic seals that protect the semiconductor from liquids and gases.

Compared with equivalent plastic packages, the TO-3 is more costly. The spacing and dimensions of the case leads make it unsuitable for higher (radio frequency) devices.

Common components that use the TO-3 package[edit]

КТ819ГМ NPN power transistor (Soviet 2N3055 copy) in TO-3 package

Common voltage regulators:

  • LM317, voltage regulator
  • LM78xx, voltage regulator
  • LM340, voltage regulator

Common transistors:

See also[edit]

  • Chip carrier Chip packaging and package types list
  • TO-18, a small metal package for low-power semiconductors
  • TO-220 plastic case used for power semiconductors with similar ratings as TO-3 cases


  1. ^

External links[edit]