How to Read FET (MOSFET) Part Numbers — 2SK · IRF · TK part numbers and 6 key datasheet numbers

Picture: A customer holds a TO-220 part marked K3569 (three legs, heat sink on top) and asks, “K3569? What is this?” The staff member raises a finger and explains, “It’s a 2SK3569. The 2S at the front has been left off.”

FET(MOSFET) is the switching component found on every power, motor and LED driver board. It is also one of the most sought-after parts when repairing older equipment.

Unlike MLCCs, FET part numbers follow different rules at every company, and many cannot tell you voltage or current from the numbers alone. Still, the common families have tricks for reading them. We have covered only those with official rules.

1. Japanese-style 2SK · 2SJ (2SA · 2SB · 2SC · 2SD)

These are part numbers registered under the Japanese standard (JIS, now managed by JEITA). The leading 2S means "3-lead semiconductor", and the third letter is the type. K = N-channel FET, J = P-channel FET, and A · B · C · D are transistors (A · B = PNP, C · D = NPN). The number after it is only the registration order, so you need the datasheet for voltage and current.

Field tip — The part body is often marked without the 2S, like K3569 or C1815. When searching, try adding 2S in front: 2SK3569 · 2SC1815

2. US-style 2N · 3N

These are part numbers registered with JEDEC in the US. 1N = diode (1N4007), 2N = 3 leads (transistors · FETs, 2N7002 · 2N3904), 3N = 4 leads (dual-gate MOSFETs, etc.). Again the number is a registration number and does not tell you the specs, but many companies make the same part number, so substitutes are easy to find: 2N7002 · 2N7000

3. IR (Infineon) IRF · IRL part numbers

In the legacy HEXFET part numbers of IR (International Rectifier, now Infineon), each letter has a meaning. IRF = standard, IRL = logic level (turns on with a gate voltage of about 5V). The next letter is the package (none means TO-220), a 9 means P-channel, and the following digit is the voltage.

Package letter — placed right after IRF (IRL)

Suffix letter

※ 4-digit part numbers such as IRF3205 · IRF4905 follow a different rule — first 2 digits 01–48 = N-channel, 49–69 = P-channel; last 2 digits = voltage (05 = 50V class, 10 = 100V). Later part numbers (IRF1010E · IRFB4227, etc.) do not follow it, so check the datasheet. Also, IRF540 · IRF740 · IRF840 without a trailing N are now Vishay products. Details: How to Read Infineon (IR) MOSFET · IGBT Part Numbers

4. Toshiba TK · TJ part numbers

Instead of 2SK · 2SJ, Toshiba uses new names starting with TK (N-channel) · TJ (P-channel). 3-pin products follow the order TK + current + package + voltage (×10) + generation, so the specs are visible from the name alone.

Package letter — placed right after the current number (Toshiba part numbering table)

5. ST STP · STB · STD part numbers

ST part numbers start with ST + package letter: STP = TO-220, STB = D²PAK, STD = DPAK (TO-252). The N in the middle means N-channel (P means P-channel), and trailing letters like NF indicate the technology generation (STripFET, etc.).

Caution — The numbers are approximate and should not be taken at face value. STP140NF75 has 140 in its name, but its continuous current is 120A, and the trailing 75 means 75V. In STD70N10F4, the trailing 10 means 100V. Treat the numbers only as a rough class and always check the datasheet.

6. 6 key numbers in the datasheet

• VDS(VDSS) — the voltage it can withstand. Leave ample margin over the circuit voltage (usually 1.5× or more).
• ID — the current it can carry. Table values are usually at 25℃, so they drop when the part actually heats up.
• RDS(on) — on-state resistance. The lower it is, the less heat. Always check at which VGS (10V · 4.5V) it was measured.
• VGS(th) — the gate voltage at which it starts to turn on. To drive it directly from a 3.3V · 5V microcontroller, it must be a logic-level type with RDS(on) specified at 4.5V · 2.5V.
• Qg — the charge needed to turn it on and off. The larger it is, the slower the switching and the harder the drive circuit has to work.
• Package · pin order — some products have a different G-D-S order even in the same TO-220.

7. How to find a substitute

Picture: A customer holds two similar-looking TO-220 parts, one in each hand, and asks, “Can I use this one as a replacement?” The staff member has two datasheets spread out on the desk, points at them with a pen and answers, “Check N-channel, pin order and voltage first.”

1. N-channel or P-channel — if they differ, it can never be used.
2. Package and pin order — whether it fits the board as is, and whether the heat sink needs insulation (full-pak or not).
3. VDS is equal or higher, and ID is equal or larger.
4. RDS(on) at the gate voltage your circuit provides is equal or lower.
5. For switching power supplies · inverters, Qg and the body diode's trr are also similar.

※ Where large currents flow, such as power stages and motor drives, always check the temperature after substituting.

Sources: JEITA (JIS C 7012) semiconductor type designations · International Rectifier Part Numbering System (Conventional HEXFET, now Infineon) · Toshiba MOSFET Part Naming Conventions · ST product pages. Companies that do not publish their rules (onsemi, Fairchild, etc.) are not included.