How to Read Transistor (BJT) Part Numbers — 2SC1815, BC547, 2N3904, MMBT3904 and SMD marking codes (1AM, 6D)

Picture: A customer looks through a magnifying glass at a tiny SMD part on a circuit board. In the magnified circle, the letters 1AM printed on the part are visible. The customer asks, “What part is 1AM?” and the female staff member smiles and answers, “It’s a marking code — MMBT3904!”

Transistor (BJT) part numbering systems differ by country. The Japanese 2SC1815, European BC547 and American 2N3904 are all similar small-signal NPN types, and in SMD form the chip carries only a 2–3 character marking code such as 1AM instead of the part number. Here is how to read them, in the order you most often encounter them in the field.

All values were checked against manufacturer datasheets (onsemi · Nexperia · Toshiba · ROHM). Click a part number to find stock in eGuro Part Search.

1. NPN·PNP and complementary pairs

How to Read Transistor (BJT) Part Numbers · Complementary Pairs

NPN and PNP come in pairs

A pair with the same shape and ratings but opposite current direction — listed as 'Complementary' in datasheets

NPN

Arrow points out

PNP

Arrow points in

C = Collector
B = Base
E = Emitter

NPN	PNP pair	Package · Ratings	Manufacturer
2SC1815	2SA1015	TO-92 · 50V 150mA	Toshiba
2N3904	2N3906	TO-92 · 40V 200mA	onsemi etc.
MMBT3904	MMBT3906	SOT-23 · 40V 200mA	onsemi etc.
BC847	BC857	SOT-23 · 100mA small-signal	Nexperia etc.
BC817	BC807	SOT-23 · 45V 500mA	Nexperia etc.
TIP31C	TIP32C	TO-220 · 100V 3A	onsemi etc.
TIP122	TIP127	TO-220 · 100V 5A Darlington	onsemi etc.
DTC114E	DTA114E	Built-in resistors · 50V 100mA	ROHM

A PNP in an NPN position will not work — a complementary part is "opposite polarity", not a "substitute"

Transistors come in NPN and PNP types depending on the direction of current flow; in the symbol, if the emitter arrow points outward it is NPN, and if it points inward it is PNP. Most have a complementary part with the same shape and ratings but opposite polarity, listed as "Complementary" in the datasheet: 2SC1815 ↔ 2SA1015, MMBT3904 ↔ MMBT3906, BC817 ↔ BC807. A complementary part has the opposite polarity and is not a substitute, so putting a PNP in place of an NPN will not work.

2. Finding part numbers from SMD marking codes (1AM·6D)

Small SMD transistors such as SOT-23 have bodies of only about 3mm, so instead of the full part number they carry only a 2–3 character marking code. For example, if you see "1AM" on a board it is onsemi MMBT3904, and "6D" is Nexperia BC817. A single character next to or after the code indicates the manufacturing date or factory and has nothing to do with the part number.

Caution: marking codes are set by each manufacturer, so the same characters may be a different part from a different company. Look at the logo and package on the body together, and check the Marking table in that company's datasheet. Package sizes are summarized in Component Package Names: A Complete Guide.

3. Japanese 2SA·2SB·2SC·2SD (2SC1815)

Japanese JIS part numbers are 2S + letter + number. The letter indicates the type: A high-frequency PNP · B low-frequency PNP · C high-frequency NPN · D low-frequency NPN. The number is a registration sequence, so you cannot derive specifications from it. The suffix O · Y · GR · BL is the gain (hFE) grade; for 2SC1815 it is O 70~140 · Y 120~240 · GR 200~400 · BL 350~700. The body is often marked without the leading 2S, as in "C1815".

4. European BC (BC547·BC817)

In the European system, the first letter = material (B silicon) and the second letter = type (C small-signal, D low-frequency power). BC546 · BC547 · BC548 are siblings that differ only in voltage (65V · 45V · 30V), and the suffix A · B · C is the hFE group (A 110~220 · B 200~450 · C 420~). The SMD versions of this family are BC846 · BC847, and the higher-current SMD types are BC817 (NPN) · BC807 (PNP) at 45V 500mA, where the suffix -16 · -25 · -40 is the hFE grade.

5. American 2N, SMD version MMBT and power TIP

2N means a three-lead semiconductor registered with the US JEDEC, and the following number is the registration number. 2N3904 (TO-92) and MMBT3904 (SOT-23) have the same 40V 200mA rating; MMBT is the part number for the same chip in an SMD package. The suffix letter often changes the rating: MMBT2222A is 40V, but MMBT2222 without the A is 30V, and for the power TIP31C, no suffix · A · B · C means 40 · 60 · 80 · 100V. TIP122 is a 5A Darlington (two transistors connected together) with a gain of about 2,500.

6. Pinout — the most common mistake

Picture: A customer holds a three-legged transistor marked C1815 in one hand and one marked 2N3904 in the other, asking, “They’re both NPN, so why doesn’t it work?” The female staff member points to a drawing on the desk. The drawing shows the legs of the C1815 as E·C·B and the legs of the 2N3904 as E·B·C. The staff member answers, “The leg order is different!”

How to Read Transistor (BJT) Part Numbers · Pinout

Even NPN parts differ in pin order

TO-92: flat side facing you, leads down, pins 1 · 2 · 3 from the left (onsemi · Toshiba datasheets). Numbers 1 · 2 · 3 in the figure = pin numbers

SOT-23SMD such as MMBT · BC8xx

1 = Base · 2 = Emitter · 3 = Collector

TO-220TIP31·TIP120

1 = Base · 2 = Collector (tab) · 3 = Emitter

Dropping a 2N3904 straight into a 2SC1815 position swaps collector and base — when substituting, rotate the leads or check the board

For leaded TO-92 transistors, the pin order differs by manufacturer family even for the same NPN type. With the flat side facing you and the leads pointing down, reading from the left, 2N3904 is emitter-base-collector (E-B-C), BC547 is collector-base-emitter (C-B-E), and 2SC1815 is emitter-collector-base (E-C-B). So if you plug a 2N3904 directly into a 2SC1815 position, the collector and base are swapped and it won't work. Most SMD SOT-23 parts are the same — 1 base · 2 emitter · 3 collector — but always check the datasheet drawing before substituting.

7. Digital transistors (DTC114E)

A digital transistor has a base resistor (R1) and a base-emitter resistor (R2) built into the body, so it connects directly to a microcontroller pin. ROHM part numbers can be read by rule: DTC114E is DT (digital) · C (NPN) · 1 (general purpose) · 14 (R1 = 1.0×10⁴ = 10kΩ) · E (R1:R2 = 1:1, so R2 is also 10kΩ). The PNP complement is DTA114E, and suffix letters such as KA · UA indicate the package. The SMD marking code is "24".

8. What to check when substituting

• Polarity — NPN or PNP (a complementary part is not a substitute)
• Pin order — TO-92 in particular mixes E-B-C · C-B-E · E-C-B
• Voltage (VCEO) · current (IC) — at least those of the original part
• hFE grade — whether the suffix (GR · B · -25, etc.) also matches
• Package — the same part number comes in several, such as TO-92 · SOT-23 · SOT-323
• If you found it by SMD marking code, whether the manufacturer logo also matches

Sources: datasheets for onsemi MMBT3904·MMBT3906·MMBT2222·MMBT5401·MMBT5550/5551·2N3903/3904·BC546~548·TIP31~32·TIP120~127, Nexperia BC817·BC807·BC846·BC856~858, Toshiba 2SC1815, ROHM DTC114E · Transistor Part Number Information (65AN139E). Marking codes and pinouts may vary by manufacturer, so check the relevant datasheet before actual use.