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BD135 NPN transistor in a TO-126 package. BD135 Transistor.jpg
BD135 NPN transistor in a TO-126 package.

TO-126 is a type of semiconductor package for devices with three pins, such as transistors. [1] The package is rectangular with a hole in the middle to allow for easy mounting to a board or a heat sink.

A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers before being diced into die, tested, and packaged. The package provides a means for connecting the package to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.

Heat sink hardware component

A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature at optimal levels. In computers, heat sinks are used to cool central processing units or graphics processors. Heat sinks are used with high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature.


History and origin

The JEDEC TO-126 descriptor is derived from the original full name for the package: Transistor Outline Package, Case Style 126. [2] In the updated JEDEC outline system, the package is numbered as TO-225AA. [3]

JEDEC standards organization

The JEDEC Solid State Technology Association is an independent semiconductor engineering trade organization and standardization body.

See also

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The TO-92 is a widely used style of semiconductor package mainly used for transistors. The case is often made of epoxy or plastic, and offers compact size at a very low cost.


The TO-220 is a style of electronic package used for high-powered, through-hole components. The "TO" designation stands for "transistor outline". TO-220 packages have three leads. Similar packages with two, four, five or seven leads are also manufactured. A notable characteristic is a metal tab with a hole, used in mounting the case to a heatsink, allowing the component to dissipate more heat than one constructed in a TO-92 case. Common TO-220-packaged components include discrete semiconductors such as transistors and silicon-controlled rectifiers, as well as integrated circuits.


In electronics, TO-3 is a designation for a standardized metal semiconductor package used for power semiconductors, including transistors, silicon controlled rectifiers, and, integrated circuits. TO stands for "Transistor Outline" and relates to a series of technical drawings produced by JEDEC.


The 2N3904 is a common NPN bipolar junction transistor used for general-purpose low-power amplifying or switching applications. The type was registered by Motorola Semiconductor in the mid-1960s, together with the complementary PNP type 2N3906, and represented a significant performance/cost improvement, with the plastic TO-92 case replacing metal cans. It is designed for low current and power, medium voltage, and can operate at moderately high speeds. This transistor is low-cost, widely available and sufficiently robust to be of use by experimenters and electronics hobbyists. When looking at the flat side with the leads pointed downward, the three wires emerging from the bottom are connected to, from left to right, the emitter, the base and the collector. Some manufacturers mark "EBC" on the molded part, but all are required to have those connections for a part which is a "2N3904".


The 2N2222 is a common NPN bipolar junction transistor (BJT) used for general purpose low-power amplifying or switching applications. It is designed for low to medium current, low power, medium voltage, and can operate at moderately high speeds. It was originally made in the TO-18 metal can as shown in the picture.

Through-silicon via

In electronic engineering, a through-silicon via (TSV) or through-chip via is a vertical electrical connection (via) that passes completely through a silicon wafer or die. TSVs are high performance interconnect techniques used as an alternative to wire-bond and flip chips to create 3D packages and 3D integrated circuits. Compared to alternatives such as package-on-package, the interconnect and device density is substantially higher, and the length of the connections becomes shorter.

In electronics, TO-18 is a designation for a style of transistor metal case. The case is more expensive than the similarly sized plastic TO-92 package. The name is from JEDEC, signifying Transistor Outline Package, Case Style 18.


DO-204 is a family of diode semiconductor packages defined by JEDEC. This family comprises lead-mounted axial devices with round leads. Generally a diode will have a line painted near the cathode end.

Chip carrier one of several kinds of surface mount technology packages for integrated circuits

In electronics, a chip carrier is one of several kinds of surface-mount technology packages for integrated circuits. Connections are made on all four edges of a square package; Compared to the internal cavity for mounting the integrated circuit, the package overall size is large.

TO-5 designation for a standardized metal semiconductor package

In electronics, TO-5 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.

Japanese Industrial Standards (JIS) has standard JIS-C-7012 for semiconductor part numbers. The first digit denotes the p-n junction count ; then follows the letter "S", then:


DO-214 is a standard that specifies a group of semiconductor packages for surface mounted diodes.


  1. BD135; BD137; BD139; NPN power transistors (PDF), Philips Semiconductors, 1999, retrieved 2013-12-09
  2. "JEDEC TO-126 package specification" (PDF). JEDEC . May 1968. Archived from the original (PDF) on June 18, 2017.
  3. AN1040/D: Mounting Considerations For Power Semiconductors (PDF), On Semiconductor, 2001, retrieved 2014-01-25