Discrete semiconductor package types fall along two axes: the device lineage they came from — diode outlines (DO, SOD, DO-214) and transistor outlines (TO, SOT) — and how they mount, through-hole or surface-mount. The package fixes three things: board area, assembly method, and thermal ceiling. It does not fix the device inside, and it does not, by itself, fix the current rating. This guide maps the whole space — diodes, BJTs, and MOSFETs — with verified dimensions, junction-to-case numbers, and real parts.

Key takeawaysTwo naming lineages: DO/SOD/DO-214 for diodes, TO/SOT for transistors and MOSFETs (JEITA/JIS adds SC names, e.g. SC-70 = SOT-323).The package is not the device. The same 2N3904 die ships in SOT-23, TO-92, and SOT-323.Surface-mount power packages carry JEDEC TO numbers: DPAK = TO-252, D2PAK = TO-263 (the SMD equivalent of the through-hole TO-220).A package sets a thermal ceiling; copper area and heatsinks decide where you land inside it — the same die dissipates 350 mW in SOT-23 but 1000 mW in SOT-223 (onsemi 2N3904 datasheet).TO-220 has a 1.5 °C/W junction-to-case path but needs a heatsink to use it: 94 W rated, only ~50 W realistic in free air (IRFZ44N datasheet).Land patterns and tab-soldering are per datasheet; power tabs and thermal pads want reflow, not hand soldering.

How discrete package names decode

Discrete packages inherited two prefixes. Diodes use DO (Diode Outline) and SOD (Small Outline Diode), plus the DO-214 surface-mount rectifier family branded SMA/SMB/SMC. Transistors and MOSFETs use TO (Transistor Outline) and SOT (Small Outline Transistor). JEDEC registers most of these; JEITA/JIS assigns parallel SC names, which is why SOT-323 and SC-70 describe the same body.

The prefix tells you lineage, not function. You cannot read a device from its package — a 2N3904 NPN ships in SOT-23, TO-92, and SOT-323, and a power MOSFET die can appear in SOT-23, DPAK, or D2PAK depending on the current target. Package selection is therefore a thermal and mechanical decision layered on top of the device you already chose.

Numbering runs opposite ways in the two families. Among diodes, a higher SOD number is usually smaller (SOD-123 > SOD-323 > SOD-523). Among transistors, the SOT numbers are model designations, not a size rank, so you read the datasheet outline rather than inferring size from the number.

Surface-mount small-signal packages (SOT and SOD)

These are the low-power workhorses: signal switching, level shifting, small logic MOSFETs, ESD diodes. They are chosen for area and cost, and most dissipate under a watt.

[IMAGE 1: SMD small-signal packages SOT-523, SC-70/SOT-323, SOT-23, SOT-23-5/6 and SOT-89 shown to scale on a 1 mm grid — alt: “Discrete semiconductor package types compared to scale: SOT-523, SC-70, SOT-23, SOT-23-5/6 and SOT-89 surface-mount bodies”]

Package	JEDEC / JEITA alias	Body / footprint (mm)	Leads	Typical P_D	Example parts
SOT-523	—	footprint ~1.8 × 1.8	3	~150 mW	small-signal BJT/MOSFET
SC-70 / SOT-323	SC-70 (JEITA)	body ~2.0 × 1.25	3–6	~200 mW	BAS40W, logic MOSFETs
SOT-23	TO-236	body 2.9 × 1.3	3	~350 mW	2N3904, BC847, 2N7002
SOT-23-5/6	—	body ~2.9 × 1.6	5–6	~350 mW	op-amps, references
SOT-89	TO-243	body ~4.5 × 2.5, heat tab	3	~1 W	medium-power BJTs, small LDOs

For scale, a SOT-523 needs only a ~1.8 × 1.8 mm footprint span, a SC-70/SOT-323 about 2.1 × 2.8 mm, and a SOT-23 about 3.1 × 3.3 mm (per JLCPCB’s assembly size data). The SOT-23 body itself is 2.9 × 1.3 mm; the SOT-23-5 widens to a 1.6 mm body for the extra pins. On the diode side, the equivalent small-signal bodies are SOD-123, SOD-323, and SOD-523 — covered in the diode-package guide linked below. SOT-89 adds a heat-spreader tab, making it more thermally capable than SOT-23 but less than SOT-223.

Surface-mount power packages (SOT-223, DPAK, D2PAK)

When a device needs to shed real power on a board without a through-hole tab, it moves to a package with an exposed metal pad soldered to copper. Numbers below come from datasheets and application notes.

Package	JEDEC alias	Body / height (mm)	θJC (°C/W)	Typical power / current	Example parts
SOT-223	TO-261	body ~6.5 × 3.5, ≤1.8 h	12	~1–2 W	LDOs, small MOSFETs
DPAK	TO-252 / SOT-428	body ~6.6 × 6.1, tab	~1–6 (die dep.)	~60 A / 150 W class	FDD86369 (80 V, 90 A)
D2PAK	TO-263 / SOT-404	body ~10 × 9, 4.5 h	~1–2 (die dep.)	~100 A / 140 W class	STB26NM60N (600 V, 20 A)
D2PAK THIN	TO-263 THIN	height 2.0 vs 4.5	~1–2	as D2PAK, lower profile	TI power devices

SOT-223 reaches a junction-to-case resistance of 12 °C/W (TI AN-1028), and its junction-to-ambient falls from about 110 °C/W to 40 °C/W as the copper pad grows from 0.0123 to 1 in² — the single clearest demonstration that copper, not the package alone, sets the limit. DPAK (TO-252) steps up to medium power: the onsemi FDD86369 is an 80 V, 90 A device rated 150 W at the junction limit, AEC-Q101 qualified. D2PAK (TO-263) is essentially the surface-mount TO-220; ST’s STB26NM60N is a 600 V, 20 A part rated 140 W. Texas Instruments offers a TO-263 THIN at 2.0 mm height versus the standard 4.5 mm where profile matters. For diodes, the SMD power equivalents are the SMA/SMB/SMC (DO-214) bodies.

Through-hole packages (TO-92 to TO-247)

Leaded packages remain the answer for high single-pulse power, heatsink mounting, socketed prototypes, and hand assembly. They dominate above roughly 50 W of continuous dissipation, where an external heatsink is unavoidable.

[IMAGE 2: Through-hole packages TO-92, TO-220 and TO-247 shown to scale with mounting tabs and holes — alt: “Discrete through-hole package types TO-92, TO-220 and TO-247 shown to scale with heatsink tabs and mounting holes”]

Package	JEDEC alias	Body (mm)	θJC / P_D	Example parts
TO-92	TO-226	~5.2 × 4.2	θJC 83 °C/W, P_D 625 mW	2N3904, MPSA92, BC547
TO-220	TO-220AB	~10 × 8.5 + tab, M3 hole	θJC 1.5 °C/W, P_D 94 W	IRFZ44N, LM7805
TO-247	TO-3P	~16 × 21 + tab	θJC <1 °C/W (large die)	high-power MOSFETs/IGBTs
TO-3	—	metal can, bolt-down	very low θJC	legacy power devices

The classic TO-92 (TO-226) small-signal body dissipates 625 mW with a junction-to-case resistance of 83 °C/W (onsemi MPSA92 datasheet). TO-220 (TO-220AB) is the power workhorse: International Rectifier’s IRFZ44N specifies a junction-to-case resistance of just 1.5 °C/W and a 94 W dissipation rating, but the same datasheet notes the package is realistically preferred “to approximately 50 watts” without aggressive heatsinking. The higher-power IRF3205 in the same TO-220 reaches a 0.75 °C/W junction-to-case path and a 200 W rating — proof that one package spans a wide power range depending on die. TO-247 and TO-3 go further for IGBTs and high-current MOSFETs. On the diode side, the axial DO-41 and DO-201AD are the through-hole equivalents.

The package sets the thermal ceiling — copper and heatsinks set where you land

This is the point the fragmented guides miss. A package name sets a ceiling on how much heat you can extract; the copper pour or heatsink you attach decides how close to that ceiling you can safely run.

The cleanest proof is one die in three packages. The 2N3904 datasheet rates the same transistor at 350 mW in SOT-23, 625 mW in TO-92, and 1000 mW in SOT-223, with junction-to-ambient resistances of 357, 200, and 125 °C/W respectively (onsemi). Nothing about the silicon changed — only the package’s ability to move heat. Layer copper on top and it improves again: SOT-223 θJA drops from ~110 to ~40 °C/W across a copper-area sweep (TI AN-1028), and even a wire-bond SOT-23 in an optimized two-layer layout can reach ~53 °C/W (TI).

Power packages make the heatsink explicit. The IRFZ44N’s 1.5 °C/W junction-to-case is only usable through its 0.50 °C/W case-to-sink path into a real heatsink; in free air the 62 °C/W junction-to-ambient is what actually limits it. The math is a short calculation: junction rise is ΔT = P_D × θ, and you keep T_J below 150 °C (or 125 °C for many automotive grades). A MOSFET dissipating 5 W on a D2PAK with a 1.5 °C/W θJC into copper stays cool; the same 5 W in a SOT-23 at 357 °C/W would demand a 1785 °C rise — impossible, so the package, not the die, is the constraint.

Footprints, standards, and compliance

The package name fixes the outline, but the land pattern, thermal-pad geometry, and via count come from the specific datasheet. Exposed tabs on DPAK, D2PAK, and SOT-223 need solid solder contact to a copper pour for their thermal numbers to hold, which is why those parts want reflow rather than hand soldering. JEDEC registers the TO and DO outlines; JEITA/JIS assigns the parallel SC designations.

For automotive designs, AEC-Q101 — “Failure Mechanism Based Stress Test Qualification for Discrete Semiconductors,” Rev E dated March 1, 2021 — governs diodes and transistors over a minimum −40 °C to +125 °C range (LEDs −40 °C to +85 °C). The DPAK FDD86369 above carries AEC-Q101 and a −55 °C to +175 °C junction range. Plastic surface-mount bodies also carry a moisture-sensitivity level (JEDEC J-STD-020) that dictates bake-and-reflow handling; leaded and hermetic parts are not moisture-sensitive. Confirm RoHS/REACH from the ordering suffix.

Worked example: a 3 A load-switch MOSFET

You need a MOSFET load switch passing 3 A continuously, surface-mount. Take a device with R_DS(on) of 25 mΩ, so conduction loss P_D = I² × R_DS(on) = 9 × 0.025 = 0.23 W. A SOT-23 at θJA ~312 °C/W would rise ~72 °C — workable but leaving no margin at elevated ambient, and SOT-23 tops out near a few hundred milliwatts. Move to SOT-223 (θJC 12 °C/W with a copper pour) and the same loss barely warms the die; move to DPAK if the current or ambient climbs. The method: compute conduction loss, add switching loss for hard-switched rails, divide the allowed T_J headroom by θ to find the copper or package you need, then verify against the datasheet’s derating curve. Start small; step up only when the thermal math fails.

Frequently asked questions

What is the difference between TO and SOT packages?

TO (Transistor Outline) is the older JEDEC family covering through-hole parts (TO-92, TO-220, TO-247) and their surface-mount descendants (TO-252/DPAK, TO-263/D2PAK). SOT (Small Outline Transistor) covers the small surface-mount bodies — SOT-23, SOT-89, SOT-223, SOT-323. In practice, SOT means small and low-power; TO spans small-signal through high-power.

What is the SMD equivalent of a TO-220?

The D2PAK (TO-263) is the surface-mount equivalent of the through-hole TO-220 — same die capability, tab soldered flat to the board instead of bolted to a heatsink. For lower power, the smaller DPAK (TO-252) is the common step down. Both were designed to move TO-220-class devices onto reflow assembly lines.

Does the package determine the current rating?

No. The package sets a thermal ceiling, and the die plus your copper or heatsink set the actual current. The same 2N3904 die is rated 350 mW in SOT-23 but 1000 mW in SOT-223 (onsemi), and SOT-223’s junction-to-ambient resistance drops from ~110 to ~40 °C/W with copper area (TI). Size the thermal path to hold the junction under its limit.

What package is a 2N3904 in?

The 2N3904 is most commonly supplied in TO-92 (through-hole) and SOT-23 (surface-mount, marked MMBT3904), and also in SOT-323 as the SC-70-sized version. It is the textbook example that one transistor die ships in multiple packages with different thermal ratings.

Is DPAK or D2PAK bigger?

D2PAK (TO-263) is larger. DPAK (TO-252) is the smaller, medium-power package; D2PAK is essentially a surface-mount TO-220 with a bigger die and higher dissipation. A rough reference: DPAK targets ~60 A-class devices, D2PAK ~100 A-class, with D3PAK (TO-268) larger still.

What does SOT stand for?

SOT stands for Small Outline Transistor, a family of compact surface-mount packages for discrete transistors, small MOSFETs, diodes, and voltage regulators. Common members are SOT-23, SOT-89, SOT-223, and SOT-323, ranging from a few hundred milliwatts (SOT-23) to a couple of watts (SOT-223) of dissipation.

The decision

Pick the device first, then the package by heat. Decode the name: DO/SOD/DO-214 is a diode, TO/SOT is a transistor or MOSFET, a leaded body is through-hole and a tabbed or gull-wing body is surface-mount. Start at the smallest package that clears your voltage and area needs, compute P_D, divide your T_J headroom by the package’s θ (and your copper or heatsink), and step up — SOT-23 → SOT-223 → DPAK → D2PAK, or TO-92 → TO-220 → TO-247 — only when the thermal math fails. The package gets you to the right shelf; the datasheet’s θ values, land pattern, and derating curve get you a board that survives.