Identifying an SMD transistor from its code
A three-lead SMD part with a short code is often a transistor, though not always. This guide goes from the code to the part number, the pinout and the meter reading that confirms it.
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Transistor, MOSFET or diode pair?
A SOT-23 body holds bipolar transistors, MOSFETs, pairs of diodes, Zener diodes, voltage references and small regulators, and the code does not say which. Some codes are printed on a switching diode by most makers and on a MOSFET by others:
Look at where the pins go before you reach for the meter. A transistor's base usually has a resistor in series with it. A MOSFET's gate usually has a resistor to its source, and a wide track runs from its drain to the load. A diode pair used as a clamp has the pin between its two diodes on a signal line and its other two on ground and on the supply.
On the meter, a MOSFET's gate reads open to both other pins, which neither a base nor a diode does. A bipolar transistor and a pair of diodes with a common pin read alike, two junctions from one pin: the datasheet's pinout settles which you have.
Common SOT-23 transistor codes
Each row puts an NPN beside the PNP it is usually paired with. The part named is the one most makers in our tables print with that code in SOT-23; the code's page lists the others.
| NPN | PNP |
|---|---|
1AMMMBT3904 14 makers, 36 parts with this code in SOT-23 | 2AMMBT3906 14 makers, 48 parts with this code in SOT-23 |
1PMMBT2222A 14 makers, 39 parts with this code in SOT-23 | 2FMMBT2907A 14 makers, 34 parts with this code in SOT-23 |
2XMMBT4401 13 makers, 33 parts with this code in SOT-23 | 2TMMBT4403 12 makers, 34 parts with this code in SOT-23 |
1DMMBTA42 11 makers, 27 parts with this code in SOT-23 | 2DMMBTA92 11 makers, 25 parts with this code in SOT-23 |
G1MMBT5551 10 makers, 35 parts with this code in SOT-23 | 2LMMBT5401 12 makers, 33 parts with this code in SOT-23 |
J3YS8050 4 makers, 11 parts with this code in SOT-23 | 2TYS8550 4 makers, 10 parts with this code in SOT-23 |
1EBC847A 12 makers, 33 parts with this code in SOT-23 | 3EBC857A 12 makers, 28 parts with this code in SOT-23 |
1FBC847B 13 makers, 35 parts with this code in SOT-23 | 3FBC857B 12 makers, 27 parts with this code in SOT-23 |
1GBC847C 12 makers, 43 parts with this code in SOT-23 | 3GBC857C 12 makers, 30 parts with this code in SOT-23 |
6ABC817-16 11 makers, 25 parts with this code in SOT-23 | 5ABC807-16 10 makers, 21 parts with this code in SOT-23 |
6BBC817-25 9 makers, 24 parts with this code in SOT-23 | 5BBC807-25 11 makers, 24 parts with this code in SOT-23 |
6CBC817-40 10 makers, 31 parts with this code in SOT-23 | 5CBC807-40 11 makers, 28 parts with this code in SOT-23 |
The last six rows are four parts, each in three gain groups. BC847A, B and C differ only in current gain, as do BC817-16, -25 and -40 and their PNP partners, and each group has a code of its own, so the code gives the gain group as well as the part.
Which pin is which
Seen from above with the single lead at the top, SOT-23 pin 1 is at the bottom left, pin 2 at the bottom right and pin 3 is the single lead.
| Pin | Bipolar transistor | MOSFET |
|---|---|---|
| 1 | base | gate |
| 2 | emitter | source |
| 3 | collector | drain |
Most bipolar transistors in SOT-23 put the base on pin 1, the emitter on pin 2 and the collector on pin 3, and most MOSFETs the gate on pin 1, the source on pin 2 and the drain on pin 3. Not every part follows this, so take the pinout from the part's own datasheet before you solder.
Of the 21 transistors printed with the codes above in SOT-23 that have a pinout in our tables, 21 put the base on pin 1, the emitter on pin 2 and the collector on pin 3.
NPN or PNP: find the base with a meter
Use the diode range, with the part lifted or at least one leg off the pad. Measure every pair of pins both ways, six readings in all.
The base is the pin that reads a junction, about 0.6 to 0.75 V, to both other pins with the same probe on it. If that probe is the red one, the transistor is NPN; if it is the black one, PNP. The other way round reads open, and collector to emitter reads open both ways.
Base to emitter usually reads a little higher than base to collector. That can tell the emitter from the collector when no datasheet is at hand. Use it as a hint, and confirm it in the datasheet when you can.
A digital transistor has resistors built in, and on the diode range a good one can read open on every pair. Measure it on the ohms range instead: base to emitter reads its two resistors in series.
Every reading, and what a failed transistor shows, is in Testing SMD parts with a multimeter.
Try it
Search 1AM to see every part printed with it and the meter test for each, then try 2A, its PNP partner.