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English Author: Site Editor Publish Time: 02-10-2026 Origin: Site
Selecting the right semiconductor package directly affects thermal performance, reliability, board layout, and long-term system stability. Among classic hermetic metal can packages, TO-5, TO-8, and TO-9 remain widely used in demanding applications where environmental resistance and predictable performance matter.
TO-series packages, short for Transistor Outline packages, originated during the early development of discrete semiconductor devices. These packages were standardized to ensure consistent mechanical dimensions, pin layouts, and mounting characteristics across manufacturers.
Metal can TO packages were originally designed to house bipolar transistors, but over time they expanded to include voltage regulators, sensors, photodiodes, laser diodes, and specialized analog components. Their longevity stems from proven reliability, standardized form factors, and excellent environmental sealing.
TO-series metal can packages share several defining characteristics:
Cylindrical metal housing, typically made from steel or Kovar
Glass-to-metal sealed leads for hermeticity
Through-hole mounting for mechanical stability
High resistance to moisture, chemicals, and temperature extremes
Predictable thermal and electrical behavior
These features make TO packages particularly valuable in harsh or mission-critical environments where plastic packages may degrade over time.
Although TO-5, TO-8, and TO-9 belong to the same family, they differ significantly in size, lead capacity, and thermal capability.
TO-5 is one of the most widely recognized metal can packages. It offers a compact footprint while maintaining hermetic sealing and moderate thermal dissipation.
Typical characteristics include:
Small body diameter
Usually 3 to 8 leads
Lightweight construction
Suitable for low-to-moderate power devices
TO-5 is commonly used for small signal transistors, precision sensors, and optoelectronic components.
TO-8 is a larger variant designed to accommodate more leads and higher power dissipation. It provides increased internal volume and surface area compared to TO-5.
Key features include:
Larger can diameter
Higher lead count options
Improved thermal handling
Greater mechanical robustness
TO-8 packages are often selected for multi-function devices or applications requiring higher power handling.
TO-9 occupies a niche position between TO-5 and TO-8 in terms of size, but its geometry supports specific device types.
Distinctive attributes include:
Medium-small form factor
Optimized for sensor and photonic devices
Balanced thermal and mechanical properties
TO-9 is frequently found in infrared sensors, gas detectors, and precision measurement components.
Physical size directly affects PCB layout, thermal dissipation, and mechanical stability. Even small dimensional differences can influence system design.
The most visible difference among TO-5, TO-8, and TO-9 packages is their body size.
Package Type | Typical Body Diameter | Typical Height | Relative Size |
TO-5 | ~9.4 mm | ~6.0 mm | Small |
TO-9 | ~8.5 mm | ~5.0 mm | Very Small |
TO-8 | ~12.5 mm | ~7.0 mm | Large |
TO-9 is often slightly smaller than TO-5 in diameter, while TO-8 is significantly larger, offering more internal space and heat dissipation surface.
Lead count influences signal routing, grounding, and device functionality.
TO-5 typically supports 3 to 8 leads
TO-9 commonly supports 2 to 4 leads
TO-8 can support 6 to 12 or more leads
Higher lead counts allow more complex circuitry or multiple sensing elements but require careful PCB layout to avoid congestion.
TO-8 packages are heavier and mechanically stronger due to thicker metal walls and larger bases. This makes them suitable for vibration-prone environments such as industrial machinery and aerospace platforms.
TO-5 and TO-9 packages are lighter, reducing mechanical stress on PCBs but offering slightly less resistance to extreme vibration.
Thermal behavior is often the decisive factor when choosing between TO-series packages.
Metal can packages dissipate heat through conduction via leads and radiation from the can surface. Larger packages generally dissipate heat more efficiently.
Package Type | Approx. Thermal Resistance (θJA) | Heat Dissipation Capability |
TO-9 | High θJA | Low |
TO-5 | Medium θJA | Moderate |
TO-8 | Low θJA | High |
TO-8’s larger surface area and mass allow better heat spreading, reducing junction temperature under load.
Typical operating characteristics include:
TO-9: low-power devices, often below 200 mW
TO-5: moderate power, typically up to 1 W with proper mounting
TO-8: higher power, often exceeding several watts with heatsinking
Metal TO packages generally tolerate operating temperatures from -55°C to +200°C, depending on device construction.
TO-8 packages are more compatible with external heatsinks or chassis mounting. TO-5 can also be heatsinked, but with lower efficiency. TO-9 rarely uses external thermal aids due to its smaller size and typical low-power operation.
Packaging affects electrical performance, especially at high frequencies or low signal levels.
Lead length and internal geometry influence parasitic inductance and capacitance.
TO-9 offers the lowest parasitics due to minimal lead count
TO-5 provides balanced performance for analog and mixed-signal devices
TO-8 may introduce higher parasitics due to longer lead paths
For RF and high-speed analog applications, TO-9 and TO-5 are often preferred.
Metal cans act as effective electromagnetic shields. All three packages provide superior EMI protection compared to plastic packages, but smaller cans generally exhibit lower internal noise coupling.
TO-9 is especially favored for low-noise sensors and photonic devices.
Hermetic sealing prevents moisture ingress, oxidation, and contamination. This ensures long-term parameter stability and low drift, making TO packages ideal for precision instrumentation and space-grade electronics.
Each TO package size aligns naturally with specific application categories.
TO-5 is widely used across multiple industries:
Precision voltage regulators
Photodiodes and optical sensors
Low-power transistors
Aerospace and defense electronics
Industrial instrumentation
Its balance of size, thermal performance, and availability makes it a versatile choice.
TO-8 is preferred when higher power or complexity is required:
Power transistors
Multi-lead sensor assemblies
Laser drivers
Industrial control modules
High-reliability power regulation
The larger form factor supports increased heat dissipation and more complex circuitry.
TO-9 specializes in sensing and detection:
Infrared sensors
Gas and chemical detectors
Temperature sensors
Optical receivers
Its compact size and low noise characteristics make it ideal for precision measurement systems.
Cost varies significantly among TO-series packages due to material usage and manufacturing complexity.
Package Type | Relative Cost | Manufacturing Complexity | Availability |
TO-9 | Low to Medium | Low | Moderate |
TO-5 | Medium | Medium | High |
TO-8 | High | High | Moderate |
TO-5 benefits from widespread industry adoption, resulting in better availability and tooling support. TO-8 costs more due to increased material and assembly requirements.
Selecting the optimal TO package involves balancing multiple factors:
Power dissipation requirements
Available board space
Number of electrical connections
Environmental exposure
Cost constraints
General selection guidelines include:
Choose TO-9 for low-power, high-precision sensing
Choose TO-5 for balanced performance and compact design
Choose TO-8 for high-power or multi-function devices
Future designs may replace TO packages with surface-mount alternatives, but for harsh environments and long-term reliability, TO-series packages remain highly relevant.