The TO-46 package is a hermetic metal can roughly 5.4 mm across the base, with three leads on a 2.54 mm pin circle and a cap short enough that it barely clears the die. JEDEC registers it as TO-206AB. It shares its header with the TO-18 and differs only in cap height.

Today it is mostly a detector can. Below are the controlled dimensions, the optical constraints that decide whether a device fits, and the capacitance ceiling that decides how fast it runs.

[IMAGE 1: three metal cans side by side — a flat-window TO-46, a ball-lens TO-46, and a TO-18 — on a millimetre scale | alt: “TO-46 package with flat window and ball lens caps compared with a taller TO-18 can”]

Key takeaways

• TO-46 is TO-206AB. Same header as TO-18; the cap is the only real difference.
• The base runs 5.309 to 5.563 mm on Analog Devices’ controlled drawing — the “5.6 mm TO can” of photonics catalogs.
• A cap aperture can be smaller than the die. Roithner’s 1 mm InGaAs detector has a 950 µm aperture.
• Junction capacitance is the bandwidth limit. 80 pF into 50 Ω puts the pole at 40 MHz.
• Detectors dominate, but the can is not optics-only: Analog Devices still ships the LM399AH precision reference in a 4-lead TO-46.

TO-46 is TO-206AB: the TO-18 base with a shorter cap

JEDEC reorganised the 2.54 mm pin-circle metal cans into the TO-206 family. TO-18 became TO-206AA, TO-46 became TO-206AB, TO-52 became TO-206AC and TO-72 became TO-206AF. The old names never left the datasheets.

What separates them is cap height, not footprint. Published figures put TO-18 at 4.83 mm, TO-52 at 3.30 mm and TO-46 at 1.90 mm. Because the header is shared, a TO-46 drops into a TO-18 footprint — the difference is how much headroom sits above the die and how much internal volume the package encloses.

That short cap is why TO-46 became the detector default. Less glass-to-die distance means less optical path to control, and a flatter cap sits closer to a lens or fibre ferrule in an assembly.

National Semiconductor’s drawings, now published by TI as SNOA033, register TO-46 in 2-, 3- and 4-lead versions under package codes H02A, H03H, H04A and H04D. A legacy datasheet naming one of those is describing a TO-46.

TO-46 package dimensions from a controlled drawing

These come from Analog Devices’ controlled outline for the 4-lead H package, LTC DWG 05-08-1341.

Feature	Inches	Millimeters
Base flange diameter	0.209–0.219	5.309–5.563
Cap diameter	0.178–0.195	4.521–4.953
Cap height above reference plane	0.085–0.105	2.159–2.667
Pin circle diameter	0.100 TYP	2.540 TYP
Lead diameter	0.016–0.019	0.406–0.483
Lead diameter, solder-dip finish	0.016–0.024	0.406–0.610
Lead length, minimum	0.500	12.700
Standoff below seating plane	0.025 max	0.635 max

One number deserves attention. The cap height on this drawing runs 2.159 to 2.667 mm, where 1.90 mm is the figure commonly quoted for TO-46. Both cannot be right for the same part. Machine any optical mount against the drawing shipped with the device you bought, not against a reference table.

The lead-diameter note matters for socket work: a solder-dip finish takes the leads up to 0.610 mm, which will bind in a cage jack sized for the 0.483 mm bare dimension.

Construction is Kovar (ASTM-F15) for matched glass seals or Alloy 52 (ASTM-F30) for compression seals, per header manufacturer EPI.

The cap aperture can be smaller than your die

Roithner’s LAPD-1-06-17-TO46, an InGaAs PIN photodiode in TO-46 with a flat glass window, is sold on a Ø1 mm active area. The same datasheet, revision 2 dated June 2022, lists the aperture diameter as 950 µm. The cap opening is smaller than the die.

For a collimated beam landing well inside the aperture, that costs nothing. For a diverging source, a wide fibre cone, or any alignment where you were counting on the full 1 mm to catch stray energy, the effective collecting area is set by the cap, not the semiconductor.

Read the aperture line, not the headline active area. Then check what the cap is: options across vendors include flat glass windows, ball lenses for fibre coupling, integrated filters, and — from Laser Components — fibre-coupled versions of every TO-46 APD they build.

Package size also caps how much die you can fit. Laser Components notes that a 1.3 mm active area fits the TO-46’s 5.3 mm base and returns about 70 % more signal than a 1 mm device in the same housing.

Junction capacitance sets your bandwidth

The TO-46 does not limit speed directly. The die area you put inside it does, through junction capacitance.

Take the same Roithner detector. Junction capacitance is 120 pF typical and 160 pF maximum at 0 V, falling to 60 pF typical and 80 pF maximum at 5 V reverse bias.

Into a 50 Ω load, the RC pole sits at:

f = 1 / (2π × 50 Ω × 80 pF) = 39.8 MHz

The datasheet quotes a 3 dB bandwidth of 30 to 40 MHz at 5 V into 50 Ω. The calculation lands squarely inside that band, which tells you the device is capacitance-limited rather than transit-time limited.

Two consequences follow. First, bias it — dropping from 0 V to 5 V halves the capacitance and roughly doubles the bandwidth for free. Second, if you need more speed, shrink the die. That is exactly what the fast parts do: Opto Diode’s silicon APDs use a Ø500 µm active area to reach a 0.5 ns rise time, and GPD’s InGaAs APDs go down to 80 µm.

[IMAGE 2: cross-section of a TO-46 can showing header, die, bond wires, cap aperture and window, with the aperture dimension called out | alt: “TO-46 package cross-section showing die, cap aperture and glass window”]

What actually ships in TO-46

Device	Type	Active area	Headline spec	Cap
Roithner LAPD-1-06-17-TO46	InGaAs PIN	Ø1 mm die, 950 µm aperture	0.90 A/W at 1.55 µm; 30–40 MHz	Flat glass
Opto Diode ODD-APD-002	Si APD	Ø500 µm	0.55 A/W at 905 nm; 0.5 ns rise; V(BR) 100–220 V	Lens top
Opto Diode ODD-APD-003	Si APD	Ø500 µm	0.36 A/W at 1064 nm; V(BR) 350–460 V	Lens top
GPD IAV008/020/035-T46	InGaAs APD	80 / 200 / 350 µm	1.0–1.63 µm spectral range	TO-46
Analog Devices LM399AH	Precision shunt reference	—	6.95 V, heated substrate	4-lead, Valox shield

The LM399AH is worth noting because it breaks the pattern. Analog Devices still lists it in a 4-lead TO-46 metal can against the same LTC drawing 05-08-1341, with a moulded thermal shield as an accessory.

On sourcing: Opto Diode’s APDs are stocked at Digi-Key, but direct orders from the manufacturer carry a $3,000 minimum. Prototype through distribution; qualify the supply route before you design in a custom cap or filter.

Four mistakes with TO-46 detectors

Designing to the active area instead of the aperture. Check both lines on the datasheet. They are not always the same number.

Leaving the detector unbiased. Junction capacitance can double at 0 V, halving your bandwidth for no reason.

Sizing a socket to the bare lead diameter. Solder-dip leads run to 0.610 mm against 0.483 mm bare.

Assuming every TO-46 cap is the same. Flat window, ball lens, filtered and open-header variants share one outline and behave completely differently in an optical path.

Frequently asked questions

What is the TO-46 package?

A hermetically sealed metal can for semiconductors, registered by JEDEC as TO-206AB. The base measures 5.309 to 5.563 mm on Analog Devices’ controlled drawing, with leads on a 2.54 mm pin circle. Three leads is standard, with 2- and 4-lead variants. It is widely used for photodiodes, avalanche photodiodes and laser diodes.

What is the difference between TO-46 and TO-18?

They share the same header and footprint. The difference is cap height — commonly quoted as 1.90 mm for TO-46 against 4.83 mm for TO-18, though Analog Devices’ TO-46 drawing gives 2.159 to 2.667 mm. A TO-46 device fits a TO-18 footprint, but the shorter cap changes the optical path and the enclosed volume.

Is TO-46 the same as a 5.6 mm TO can?

In practice, yes. Photonics catalogs name TO headers by base diameter, and the TO-46 base measures 5.309 to 5.563 mm. Laser diode and photodiode suppliers list TO-46 alongside Ø3.8 mm, Ø5.6 mm and Ø9 mm cans, with the 5.6 mm entry corresponding to this outline.

What is the TO-46 package used for?

Mostly optical detectors and emitters: silicon and InGaAs PIN photodiodes, avalanche photodiodes for LiDAR and rangefinding, and laser diodes. It is also still used for precision analogue parts — Analog Devices ships the LM399AH voltage reference in a 4-lead TO-46 with a moulded thermal shield.

What to do next

If you are specifying a TO-46 detector, read three lines before anything else: aperture diameter, junction capacitance at your intended bias, and cap type. Those set your real collecting area, your bandwidth ceiling and your optical interface. The active-area headline sets none of them on its own.

If you are laying out a footprint, draw it to a 2.54 mm pin circle and confirm the lead finish, because solder-dip parts will not enter a socket sized for bare leads. And if the design needs a non-standard window, filter or fibre pigtail, raise it with the supplier early; custom caps are routine at these vendors but they change lead time.

External references

• Analog Devices 4-lead TO-46 outline, LTC DWG 05-08-1341
• Roithner LAPD-1-06-17-TO46 InGaAs photodiode datasheet, rev.2 06/2022
• Texas Instruments SNOA033, Metal Can Packages
• Analog Devices LM199 / LM399 precision reference datasheet
• Opto Diode APD series technical summary

Proposed internal links

• [INTERNAL LINK: TO-39 package → TO-39 package dimensions and pinout]
• [INTERNAL LINK: TO-18 package → TO-18 package guide]
• [INTERNAL LINK: photodiode bandwidth → junction capacitance and transimpedance amplifier design]
• [INTERNAL LINK: hermetic packages → hermetic versus plastic semiconductor packaging]
• [INTERNAL LINK: JEDEC transistor outlines → JEDEC JEP95 outline naming explained]