TO-5, TO-18, and TO-39: What’s the Difference and Where to Use Each Package

In the world of semiconductors, packaging is far more than a protective shell — it is a vital component that determines how efficiently a device performs, dissipates heat, and survives harsh environments. Among the many standardized metal can packages developed for transistors, sensors, and optoelectronic components, three classic formats stand out: TO-5, TO-18, and TO-39.

Despite being introduced decades ago, these Transistor Outline (TO) packages remain the backbone of military, aerospace, and industrial electronics, where reliability and hermeticity are critical. This article explores their origins, structures, and performance differences, offering practical guidance on how to select the best package for your application.

Understanding Transistor Outline (TO) Packages

The term “TO” (Transistor Outline) was established by the Joint Electron Device Engineering Council (JEDEC) to define standardized package outlines for transistors and diodes. TO packages typically use metal cans with glass-to-metal feedthroughs to hermetically seal the semiconductor die from moisture, gas, and mechanical stress.

Key CharacteristicsHermetic sealing: protects the die from oxidation, humidity, and contaminants.

• Excellent thermal conductivity: metal enclosure provides efficient heat dissipation.

• Mechanical robustness: suitable for aerospace and defense where vibration resistance is required.

  Standardized dimensions: simplifies cross-manufacturer compatibility.

While plastic encapsulated packages dominate consumer electronics, TO metal cans remain irreplaceable in mission-critical systems such as precision sensors, photodiodes, and high-temperature power transistors.

Overview of TO-5, TO-18, and TO-39 Packages

The TO-series packages are visually similar but differ significantly in size, power capacity, and pin configuration. The following table summarizes their basic features.

The TO-18 is the smallest and most compact, optimized for miniature low-power devices. The TO-5 offers more pin flexibility and space for complex circuits. The TO-39 shares the same footprint as TO-5 but features a thicker metal wall and larger die attach area, allowing higher heat dissipation.

Structural and Material Differences

Physical Construction

All three package types consist of the same essential components:

• Header/Base: Typically composed of Kovar, a nickel-cobalt-iron alloy that matches silicon and glass in terms of regulated coefficient of thermal expansion (CTE).

• Glass feedthroughs: Provide hermetic electrical isolation between leads and the metal base.

• Metal can or cap: Often nickel-plated steel or Kovar, welded or sealed to the header.

• Leads: Typically nickel or tin-plated for solderability.

Size and Capacity

• TO-18: Small, lightweight, ideal for space-constrained designs. However, its limited thermal mass restricts power dissipation.

• TO-5: Offers a wider cavity and supports up to eight leads, enabling multi-chip assemblies or hybrid modules.

• TO-39: Shares the TO-5 base size but uses a taller and heavier cap with thicker walls for better heat flow and mechanical strength.

Hermeticity and Reliability

Metal cans provide long-term hermetic protection that far surpasses plastic encapsulants. They meet MIL-STD-883 and MIL-PRF-19500 standards, ensuring survivability in radiation, vacuum, and extreme temperature environments — essential for defense and aerospace missions.

Electrical and Thermal Performance

The electrical and thermal characteristics of TO packages are determined by their geometry and materials. Larger packages with thicker metal bodies naturally have lower thermal resistance and higher power-handling capability.

• TO-18 is suitable for low-power signal circuits where minimal heat is generated.

• TO-5 supports moderate power dissipation while maintaining hermetic sealing.

• TO-39 is engineered for high-temperature and high-current environments, with superior heat conduction and mechanical stability.

Typical Applications

TO-18

• Small signal transistors: e.g., 2N2222A, 2N2907A.

• Optoelectronic sensors: photodiodes, phototransistors, and laser diodes.

• Temperature and pressure sensors: where miniaturization and low mass are critical.

• Medical instrumentation: ECG amplifiers, implantable circuits.

TO-5

• Operational amplifiers and hybrid ICs: LM741, precision analog modules.

• Gas sensors and radiation detectors: where multiple lead configurations are needed.

• Analog optoelectronic modules: LED–photodiode pairs, optical couplers.

• Aerospace instrumentation: used in high-stability signal chains and amplifiers.

TO-39

• Power transistors and voltage regulators: 2N3053, LM317.

• Laser diodes and photodetectors: providing robust thermal paths.

• RF amplifiers: for transmitters, radar modules, and RF test equipment.

• High-temperature sensors: suitable for under-hood automotive or oil-well logging environments.

Design Considerations and Mounting

Mounting Techniques

TO packages are designed primarily for through-hole mounting. Leads can be soldered directly into PCBs or sockets. For high-reliability applications, press-fit or weldable headers are often used.

Some high-power TO-39 devices can be mounted on heat sinks or metal chassis, with thermal compound to enhance conduction. The can itself is sometimes connected to ground for EMI shielding.

Handling Precautions

• Avoid excessive bending of leads near the glass seal to prevent cracking.

• Use ESD protection when handling bare transistors or sensors.

• During soldering, maintain controlled heating to protect the hermetic seal.

• Ensure correct pin orientation — TO packages often include an alignment tab or locating dot.

Design Integration

Because of their cylindrical form, TO packages occupy more vertical height than flat plastic SMD packages. When integrating them into modern PCBs:

• Maintain adequate clearance for the metal can.

• Consider vibration and thermal expansion factors in mechanical design.

• For high-frequency circuits, minimize lead length to reduce parasitic inductance.

Cost and Availability

While metal-can packages are more expensive than plastic encapsulated SMD types, their cost is justified in mission-critical applications.

Pricing Trends

• TO-18: lowest cost (~$0.20–0.80 USD in bulk), widely available from brands like ON Semiconductor and Vishay.

• TO-5: moderate cost (~$0.50–1.50 USD), used in specialized analog circuits.

• TO-39: highest among the three (~$1.50–3.00 USD), due to thicker metal and higher thermal specs.

Supply and Sourcing

Manufacturers such as Texas Instruments, Vishay, Microchip, Central Semiconductor, and Hamamatsu still offer active production. Hermetic TO packages are also available from specialized suppliers serving aerospace, medical, and photonics industries.

Lead time is typically 6–12 weeks for standard parts and longer for custom pin configurations or nickel-gold finishes.

Choosing the Right Package

Selecting between TO-18, TO-5, and TO-39 depends on your power, size, and environmental requirements. The following succinctly describes the decision-making process:

Define Power Requirements:

• Below 0.5 W → TO-18.

• 0.5–1 W → TO-5.

• Above 1 W or >150 °C operation → TO-39.

Consider Pin Count and Functionality:

• For simple 3-pin transistors → TO-18 or TO-39.

• For multi-lead hybrids, sensors, or op-amps → TO-5.

Assess Environmental Stress:

• For vibration, radiation, or vacuum conditions → choose hermetic TO-5 or TO-39.

• For compact portable instruments → TO-18 balances performance and size.

Evaluate Cost and Size Trade-off:

• TO-18 is economical for high-volume low-power devices.

• TO-5/TO-39 deliver long-term stability for precision and high-reliability systems.

Selection Example

If you are designing a laser-based distance sensor, the photodiode might use TO-18 for compactness, while the laser driver transistor may require TO-39 for better heat dissipation. In a gas detection module, a TO-5 header is ideal since it supports multiple sensor pins and gas-exposure windows.