science

TO-3

TO-3 is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand TO-3 rather than just read about it. In short: 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.

Key takeaways

TO-3 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TO-3 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TO-3 from memory before moving on to harder problems.

Reference excerpt

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 TO-3 case has a flat surface which can be attached to a heatsink, normally via a thermally conductive but electrically insulating washer. The design originated at Motorola around 1955 from a group headed by Dr. Virgil E. Bottom. who was director of research of the Motorola Semiconductor Division. The first use of this design was for the germanium alloy-junction power transistor 2N176 – the first power transistor to be put into quantity production. The lead spacing was originally intended to allow plugging the device into a then-common tube socket.

Typical applications

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 of heat dissipation. The case surfaces are metal for good heat conductivity and durability. The metal-to-metal and metal-to-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 frequency (radio frequency) devices.

Construction

The semiconductor die component is mounted on a raised platform on a metal plate, with the metal can welded on top of it; providing high heat conductivity and durability. The metal case is connected to the internal device and the leads are connected to the die with bonding wires. The TO-3 package consists of a diamond-shaped base plate with diagonals of 40.13 mm (1.580 in) and 27.17 mm (1.070 in). The plate has two mounting holes on the long diagonal, with the centers spaced 30.15 mm (1.187 in) apart. The cap attached to one side of the plate brings the total height to up to 11.43 mm (0.450 in). Two pins on the other side of the plate are isolated from the package by individual glass-metal seals. The metal case forms the third connection (in the case of a bipolar junction transistor this is typically the collector).

Dimensions

Variants

TO-3 package variants for integrated circuits can have more than two leads. The height of the cap and the thickness of the leads differs between variants of the TO-3 package.

TO-41

The two pins of the TO-41 package end in soldering pads with holes in them to make it easier to solder wires to the pins for point-to-point construction (as opposed to soldering a TO-3 package on a printed circuit board). Otherwise the TO-41 package has the same dimensions as the TO-3 package. Some variants of the TO-41 package have a third pin with a soldering pad connected to the case (e.g. AD133, AUY21). This 3-pin package was standardized by IEC as C14B/B28.

TO-204 TO-204 is intended to replace previous definitions of flange-mounted packages with a 10.92 mm (0.430 in) pin spacing. The different outlines are now defined as variants of TO-204: TO-3 is renamed to TO-204-AA, TO-41 to TO-204-AB. A new package with a reduced maximum height of 7.62 mm (0.300 in) is added as TO-204-AC. Two additional variants specify pins thicker than the original 1.02 mm (0.040 in) to allow higher currents: 1.27 mm (0.050 in) for TO-204-AD and 1.52 mm (0.060 in) for TO-204-AE.

National standards

Common components in a TO-3 package Common voltage regulator integrated circuits:

LM317, voltage regulator LM78xx, voltage regulator LM340, voltage regulator Common transistors:

2N3055, NPN power transistor MJ2955, PNP power transistor (not to be confused with a 2N2955 which is a small signal PNP transistor ) KD503, NPN power transistor

See also TO-66, smaller package of similar shape TO-220 plastic case used for power semiconductors with similar ratings to TO-3 cases

References

External links

TO-3 standard from JEDEC TO-3 package from EESemi.com Hermetic packages from National Semiconductor

Illustrations

Typical TO-3 mounting profile, with insulator from chassis

Worked examples

Example 1 — a first encounter with TO-3

Start with the simplest possible case. Write down what TO-3 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TO-3 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TO-3 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TO-3

In research: TO-3 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TO-3 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TO-3 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Semiconductor packages, so understanding it makes those chapters shorter.
In everyday life: Look for TO-3 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study TO-3 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TO-3 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TO-3 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TO-3 in simple terms?

Why does TO-3 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TO-3?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TO-3.