Transistor package types determine how a discrete semiconductor connects to your PCB, dissipates heat, and survives its operating environment. The two dominant families are TO (Transistor Outline) for through-hole and power applications and SOT (Small Outline Transistor) for surface-mount designs. Power SMD packages like DPAK and D2PAK bridge the gap. This guide covers every package you will encounter, compares them in a single parametric table, and gives you a clear decision path for your next design.

Key Takeaways

TO-92 is the default through-hole small-signal package. It dissipates ≤625 mW in free air (RθJA ≈ 200 °C/W per ROHM 65AN132E) and houses classics like the 2N3904 and BC547.

TO-220 handles 50–100+ W with a heatsink (RθJC ≈ 1–3 °C/W). Without a heatsink it safely dissipates only about 1 W at 25 °C ambient.

SOT-23 is the most widely used SMD transistor package: 2.9 × 1.3 mm body, 0.95 mm lead pitch, JEDEC designation TO-236AB. It covers signal switching up to ~600 mA.

SOT-223 can replace DPAK on the same footprint with 25% lower profile and 35% shorter length, per the Infineon CoolMOS app note, while maintaining comparable thermal performance when paired with ≥50 mm² of copper pour.

AEC-Q101 is the qualification standard for transistors in automotive applications. It mandates stress testing from −40 °C to +125 °C minimum. Always confirm AEC-Q101 status on the datasheet for under-hood designs.

TO — Transistor Outline Packages

JEDEC’s Transistor Outline designations cover the broadest range of discrete semiconductor packages, from milliwatt signal devices to multi-hundred-watt power modules. The “TO” prefix is followed by a serial number assigned by JEDEC; the numbers themselves carry no dimensional meaning. Each TO designation also has a formal JEDEC JEP-95 name (e.g., TO-92 is officially TO-226AA), though the informal names dominate in practice.

TO-92 — The Universal Small-Signal Package

The TO-92 is a three-lead plastic-molded through-hole package with a flat face for marking and a semi-circular back. Lead pitch is 1.27 mm (50 mil). Body dimensions are approximately 4.2 mm × 3.8 mm × 5.3 mm tall. ROHM’s app note 65AN132E lists RθJA at 625 °C/W on a minimal 12 mm × 20 mm FR4 board. This limits practical free-air dissipation to roughly 200 mW at 25 °C ambient with a 150 °C junction limit.

Common transistors in TO-92: 2N3904, 2N3906 (NPN/PNP pair), BC547/BC557, 2N7000 (N-channel MOSFET). The SMD equivalent is SOT-23.

TO-220 — The Workhorse Power Package

TO-220 (JEDEC TO-220AB) is the most common through-hole power package. It features a metal tab with an M3 mounting hole for heatsink attachment. Pin spacing is 2.54 mm (100 mil). The Analog Devices thermal table lists RθJA at ≈50–65 °C/W in free air and RθJC at approximately 1–3 °C/W depending on die size.

The TO-220 is rated for devices handling less than 100 A and operating below a few hundred volts. Typical inhabitants include the IRFZ44N MOSFET, TIP31/41 BJTs, and the LM7805 voltage regulator. STMicroelectronics’ AN1703 reports RθJA of 62.5 °C/W for the TO-220, identical to the SMD D2PAK in free air.

TO-247, TO-264 and TO-3 — High-Power Territory

TO-247 (JEDEC TO-247AC) is the step up from TO-220: a larger plastic body of 15.8 mm × 21.0 mm with lead pitch of 5.45 mm and RθJC of 0.5–1.5 °C/W. ROHM 65AN132E lists RθJA at 30 °C/W for through-hole mounting. Devices in TO-247 routinely handle 150–250+ W with adequate heatsinking.

TO-264 is even larger (20.2 mm × 26.0 mm body), serving IGBTs and SiC MOSFETs in the 200–300+ W range. The legacy TO-3 (JEDEC TO-204AA) is an all-metal hermetically sealed can using 6-32 screw mounting, still found in military and aerospace power supplies but rarely specified in new commercial designs.

TO-126 — Mid-Power, Often Overlooked

TO-126 (JEDEC TO-225AA, also called SOT-32 by STMicroelectronics) sits between TO-92 and TO-220. It provides a small heatsink tab and dissipates roughly 10–20 W with a clip-on heatsink. RθJC is typically 5–10 °C/W. It was once common for VAS (voltage amplifier stage) transistors in audio amplifiers but has been largely displaced by SOT-223 and DPAK in new designs.

[IMAGE 1: Side-by-side photo of TO-92, TO-126, TO-220, and TO-247 packages with key dimensions labeled | alt: “TO-92 TO-126 TO-220 and TO-247 transistor package types compared by size”]

SOT — Small Outline Transistor Packages

SOT packages are surface-mount plastic-molded housings originally developed to replace through-hole TO-92 and TO-18 packages in consumer electronics. Despite the “transistor” name, SOT packages also house diodes, voltage regulators, and simple ICs. JEDEC assigns formal designations (e.g., SOT-23 = TO-236AB), but manufacturers add their own codes—SOT-23 alone has at least 10 synonyms including SC-59 (EIAJ), Micro3 (International Rectifier), and SMT3 (ROHM), per mbedded.ninja.

SOT-23 — The Default SMD Transistor Package

SOT-23-3 is the most produced SMD transistor package worldwide. Body dimensions: 2.9 × 1.3 mm (SOT-23-5 and -6 variants widen to 1.6 mm). Lead pitch: 0.95 mm. ROHM 65AN132E lists RθJA at 625 °C/W on a minimal board—roughly the same as TO-92, so practical dissipation without copper pour is limited to ~200 mW. Continuous current capability is typically ≤600 mA.

SOT-23 handles NPN/PNP BJTs, small MOSFETs, Schottky diodes, and single-gate logic. The 3-lead variant dominates; 5-lead (SOT-23-5) and 6-lead (SOT-23-6) versions serve op-amps, comparators, and voltage references.

SOT-89 and SOT-223 — Medium-Power SMD

SOT-89 (JEDEC TO-243AA) adds a metal heat-dissipation tab connected to the collector (for BJTs) or drain (for FETs). Current capability reaches ~1 A. The tab is on the bottom of the package body, meaning PCB copper directly underneath determines thermal performance.

SOT-223 (JEDEC TO-261AA) has a larger exposed tab and 2.3 mm lead pitch. ROHM’s app note on SOT-223-3 replacing DPAK shows that with ≥50.6 mm² of copper foil, SOT-223 achieves thermal resistance equivalent to DPAK (152.6 °C/W). Infineon’s CoolMOS app note confirms SOT-223 is 25% lower in height and 35% shorter than DPAK, and the leads fit directly on the DPAK footprint for drop-in replacement.

SOT-323 (SC-70) and SOT-523 — Miniature Packages

SOT-323 measures roughly 2.0 × 1.25 mm with a 0.65 mm lead pitch. SOT-523 shrinks further to ~1.6 × 0.8 mm with a 0.5 mm pitch. Both carry ≤200 mA and are intended for automated assembly only. Manual rework of SOT-523 is not practical in production. Reserve these for high-density digital switching where current demands are low.

[IMAGE 2: Top-down footprint comparison of SOT-523, SOT-323, SOT-23, SOT-89, SOT-223, DPAK, and D2PAK on a 1 mm PCB grid | alt: “SOT and DPAK transistor package footprint comparison on PCB grid”]

Power SMD Packages — DPAK and D2PAK

DPAK (TO-252, Decawatt Package) and D2PAK (TO-263, Double Decawatt Package) bridge the gap between SOT-class SMD packages and through-hole power devices. Both use large exposed pads soldered directly to PCB copper for thermal transfer.

DPAK (TO-252): footprint span ~6.6 × 11.2 mm. STMicroelectronics AN1703 reports RθJA of 100 °C/W in free air, dropping to 42 °C/W with a 120 mm² copper pad. Handles up to ~2–3 A continuous in typical designs.

D2PAK (TO-263): footprint span ~10.4 × 17.0 mm. RθJA of 62.5 °C/W in free air per ST AN1703—matching TO-220 in free air. Handles tens of amps with proper copper and thermal vias. Motorola originally designed the TO-263 as a surface-mount equivalent to the TO-220.

Transistor Package Types at a Glance

Package	JEDEC Name	Mount	Leads	Pitch	Body Size (typ.)	RθJA (°C/W)	Typical Pᵈ Range
TO-92	TO-226AA	THT	3	1.27 mm	4.2×3.8 mm	625*	≤200 mW
TO-126	TO-225AA	THT	3	2.54 mm	8.5×6.6 mm	≈70	10–20 W
TO-220	TO-220AB	THT	3–7	2.54 mm	10×15 mm	≈62	50–100+ W†
TO-247	TO-247AC	THT	3–4	5.45 mm	15.8×21 mm	30	150–250+ W†
TO-3	TO-204AA	THT	2+case	6.7 mm	Ø31 mm can	≈35	100–250 W†
SOT-23	TO-236AB	SMD	3–6	0.95 mm	2.9×1.3 mm	625*	≤200 mW
SOT-323	SC-70	SMD	3–6	0.65 mm	2.0×1.25 mm	≈780	~100 mW
SOT-89	TO-243AA	SMD	3	1.5 mm	4.5×2.5 mm	≈200	0.5–1 W
SOT-223	TO-261AA	SMD	3–4+tab	2.3 mm	6.5×3.5 mm	≈160‡	0.5–2 W
DPAK	TO-252	SMD	3	2.29 mm	6.5×6.1 mm	100 / 42§	1–3 W
D2PAK	TO-263	SMD	3–7	2.54 mm	10.2×9.9 mm	62.5	2–50+ W

* On 12×20 mm FR4 per ROHM 65AN132E. † With heatsink; free-air ~1 W. ‡ On min. recommended footprint. § Free air / with 120 mm² pad per ST AN1703. All RθJA values are representative—consult specific device datasheets.

How to Choose a Transistor Package

Start with three questions in this order:

1. How much power must the device dissipate? Estimate Pᵈ = Vᶜᵉ × Iᶜ (BJT) or Iᵈ² × Rᵈₛₒₙ (MOSFET). If Pᵈ < 200 mW, SOT-23 or TO-92 suffices. At 200 mW–1 W, consider SOT-89 or SOT-223 with adequate copper. At 1–10 W, DPAK or TO-220 without a heatsink. Above 10 W, TO-220 or TO-247 with a dedicated heatsink.

2. Through-hole or surface mount? If your board is fully SMT (reflow-only), eliminate TO-92, TO-220, and TO-247 unless you can add a selective-solder step. SOT-223 or DPAK handle moderate power within an SMD-only flow. If you already have through-hole connectors requiring wave solder, adding a TO-220 costs nothing extra.

3. How much board area can you spare? SOT-23 occupies ~7.5 mm² of footprint. DPAK occupies ~73 mm². D2PAK uses ~177 mm². If space is the primary constraint and power is low, SOT-323 or SOT-523 shrinks the footprint below 4 mm², but requires automated placement.

Thermal Design Essentials

Every transistor package datasheet states RθJA (junction-to-ambient thermal resistance), but this number is measured on a specific test board and changes dramatically with your PCB layout. For SMD packages with exposed pads (SOT-89, SOT-223, DPAK, D2PAK), the copper pour area underneath the pad dominates thermal performance.

ROHM’s SOT-223-3 app note (66AN023E) demonstrates that increasing the copper foil area from 20 mm² to 50.6 mm² reduces the SOT-223’s thermal resistance from above 200 °C/W down to the DPAK’s 152.6 °C/W. The practical rule: always provide at least the manufacturer’s recommended copper pad area. For multilayer boards, add thermal vias (0.3–0.4 mm drill, 4–9 vias in a grid) under the exposed pad to conduct heat to inner planes.

For through-hole power packages (TO-220, TO-247), the junction-to-case resistance RθJC is the critical parameter. The thermal model is: Tⱼ = Tₐ + (RθJC + RθCS + RθSA) × Pᵈ. The case-to-sink resistance RθCS depends on the interface material: bare contact ~1 °C/W, thermal grease ~0.3–0.5 °C/W, silicone pad ~0.5–1.5 °C/W, mica washer with grease ~0.5–1.0 °C/W (per ON Semiconductor AN1040).

Naming Confusion — JEDEC, EIAJ and Manufacturer Codes

Transistor packages carry different names depending on who is writing the datasheet. The same physical package may appear under JEDEC, EIAJ/JEITA, and proprietary manufacturer designations. The table below cross-references the most common packages.

Common Name	JEDEC (JEP-95)	EIAJ/JEITA	Other Names
TO-92	TO-226AA	—	SOT-54 (Philips legacy)
TO-220	TO-220AB	SC-46	SOT-78 (NXP)
TO-126	TO-225AA	—	SOT-32 (ST)
TO-247	TO-247AC	—	—
SOT-23	TO-236AB	SC-59	Micro3 (IR), SMT3 (ROHM)
SOT-89	TO-243AA	SC-62	SOT-89-3 (many mfrs)
SOT-223	TO-261AA	SC-73	KC-3 (Analog Devices)
DPAK	TO-252	SC-63	SOT-428 (formed variant)
D2PAK	TO-263	SC-83	SOT-404, DDPAK

Sources: JEDEC JEP-95 package outline index, EIAJ/JEITA ED-7500A, Wikipedia, mbedded.ninja. “—” means no distinct designation exists or the package predates the standard.

[IMAGE 3: Flowchart: Power budget → Mount type → Board area → Package recommendation | alt: “Transistor package type selection flowchart for PCB designers”]

Frequently Asked Questions

What does TO mean in transistor packages?

TO stands for Transistor Outline, a JEDEC designation system for discrete semiconductor packages. The number following TO (e.g., TO-92, TO-220) is a serial identifier assigned by JEDEC; it does not encode the package’s dimensions or pin count. JEDEC also assigns formal names like TO-226AA for TO-92.

What is the most common transistor package?

For through-hole designs, the TO-92 dominates small-signal work and the TO-220 dominates power applications. For surface mount, the SOT-23 is the single most produced transistor package worldwide, used for BJTs, small MOSFETs, and diodes. SOT-23-3 alone has more active part numbers than any other discrete package.

What is the difference between TO-220 and DPAK?

TO-220 is through-hole with a bolt-on heatsink tab; DPAK (TO-252) is surface mount with a soldered exposed pad. Both have similar RθJA in free air (≈62–100 °C/W). TO-220 achieves far lower thermal resistance with an external heatsink (RθJC ≈1–3 °C/W), while DPAK relies on PCB copper for heat spreading. Choose TO-220 for high-power, DPAK for SMD-only assembly.

Can SOT-223 replace DPAK?

Yes, in many cases. The SOT-223 leads fit directly on the DPAK footprint. Infineon’s CoolMOS app note confirms that SOT-223 operates within 2.8 °C of DPAK at 0.25 W dissipation. With ≥50 mm² of copper pour, SOT-223 matches DPAK’s thermal performance while being 25% lower in height. For high-current designs above ~3 A, DPAK or D2PAK remains necessary.

What is AEC-Q101 for transistors?

AEC-Q101 is the Automotive Electronics Council’s stress-test qualification standard for discrete semiconductors including transistors, MOSFETs, and diodes. It mandates testing across −40 °C to +125 °C minimum and covers HTOL, temperature cycling, ESD, moisture resistance, and more. Any transistor used in an automotive ECU, inverter, or sensor module should carry AEC-Q101 qualification.

Making the Call — What to Specify Next

Match your power budget to the smallest package that meets it. For signal switching under 200 mW, SOT-23 is the default unless you need breadboard access (then TO-92). For moderate power up to 2 W, SOT-223 with copper pour gives you a compact, heatsink-free solution and can drop onto existing DPAK footprints. For 2–50 W, DPAK or D2PAK keeps you in SMD territory; above 50 W, TO-220 or TO-247 with an external heatsink is the proven path.

Before finalizing, verify three things: (1) that the specific part number is still in active production in your chosen package—many BJTs have dropped TO-92 or TO-126 options, (2) that the package carries the required qualification (AEC-Q101 for automotive, MIL-PRF-19500 for defense), and (3) that your PCB layout provides the minimum copper area stated in the datasheet’s thermal management section. Undersizing the pad is the single most common cause of thermal failures in SOT-89 and SOT-223 designs.