SMD Transistor Marking Codes: SOT-23, SOT-223, SC-70 and SOT-89

A small SMD transistor never says “BC847B” on top. It says 1F, or K1R, or SS. Resistor codes follow a standard, so 103 means 10 kΩ no matter who made the part. Transistor and small-IC codes do not: each manufacturer assigns its own. This page explains what the characters mean, how to get from a code to a datasheet, which pin is which, and how to test the part with a multimeter.

Packages you will meet

• SC-70 (also SOT-323): the smallest of the group, about 2.0 × 1.25 mm, three leads.
• SOT-23: the most common, about 2.9 × 1.3 mm, two leads on one side and one on the other. Five- and six-lead versions (SOT-23-5, SOT-23-6) usually hold ICs such as regulators and op-amps; see SMD IC marking.
• SOT-89: about 4.5 × 2.5 mm, three leads on one side, the middle one wider and joined to the pad under the body.
• SOT-223: 6.5 × 3.5 mm, three narrow leads at the bottom and a wide fourth lead (tab) at the top for heat.

The package alone tells you little. A SOT-23 may contain a bipolar transistor, a MOSFET, a dual diode, a Zener or a voltage reference. Diodes in these packages are covered in SMD diode marking.

Why a code instead of a part number

There is room for three or four characters on a SOT-23, and a part number like MMBT3904 needs eight. So the maker prints a short device code, often followed by a date or factory character.

What the extra characters mean is defined in each datasheet:

• onsemi uses XXMG: XX is the device code, M a date code, G or a microdot marks a lead-free part. So 1F plus a letter is the code 1F plus a date.
• Diodes Incorporated prints XXX with YM next to it: year as a letter, then the month.
• Philips (later NXP, now Nexperia), in the 2002 BC847 datasheet, used 1F*, where * is the factory: p for Hong Kong, t for Malaysia.

Lesson one: not every character is part of the code. Search for the full string and for the first two characters.

Same transistor, different codes

There is no common standard for these codes. The same device carries different codes depending on the manufacturer, straight from their own datasheets:

Device	Type	onsemi	Diodes Inc.	Philips (2002)
BC847B	NPN, 45 V, 100 mA	1F	K1R	1F*
MMBT3904	NPN, 40 V, 200 mA	1AM	K1N	—
BSS138	N-channel MOSFET, 50 V	SS	K38	—

A dash means we did not check it. It also works the other way: one code, several devices. Diodes marks BC846B, BC847B and BC848B (65, 45 and 30 V parts) all as K1R, and the A versions all as K1Q. onsemi uses 1F for BC847B in SOT-23 and for BC847BW in SC-70; only the package size tells them apart.

Gain groups: 1E, 1F, 1G

The A/B/C suffix of BC847 is the hFE (current gain) group, and the code follows it:

Device	hFE at 2 mA	onsemi, Philips	Diodes
BC847A	110–220	1E	K1Q
BC847B	200–450	1F	K1R
BC847C	420–800	1G	K1M

How to identify a part from its code

1. Read every character under magnification and write it down. Keep in mind the last one may be a date.
2. Identify the package and lead count. Measure it: SC-70 and SOT-23 are easy to confuse by eye.
3. Look at the circuit around it. A base resistor and a load suggest a switch; caps on input and output suggest a regulator; parts next to a connector are often protection diodes.
4. Look up candidates in an SMD code book or online code database by code and package. Expect several devices from different makers.
5. Open each candidate’s datasheet and check the marking table (Marking, Ordering Information) and the pinout.
6. Confirm by measurement. If you have the service schematic of the unit, the part number there beats any code book.

SOT-23 pinout

Numbering is always the same: top view, two leads at the bottom — 1 on the left, 2 on the right — and the single lead opposite is 3. What sits on each pin depends on the device:

Variant	1	2	3	Checked in
Bipolar, most common	base	emitter	collector	onsemi BC847, MMBT3904; Philips BC847
MOSFET, most common	gate	source	drain	onsemi and Diodes BSS138
Other bipolar	emitter	base	collector	onsemi SOT-23 outline, STYLE 7
Other MOSFET	drain	source	gate	onsemi SOT-23 outline, STYLE 10

Base–emitter–collector is the usual layout, not a rule. onsemi’s SOT-23 package drawing alone lists more than twenty pin styles. Take the pinout from the datasheet of the exact part.

SOT-223 and SOT-89: the wide lead

The wide top lead of a SOT-223 (pin 4, or TAB) moves heat into the copper. In every example we checked it is connected to the middle pin 2, but what pin 2 does varies.

Device, package	1	2	3	4 (TAB)
BCP56 (onsemi), SOT-223	base	collector	emitter	collector
LM1117 (TI), NCP1117 (onsemi), SOT-223	ADJ/GND	output	input	output
AZ1117C (Diodes), SOT-223, H option	ADJ/GND	output	input	output
AZ1117C (Diodes), SOT-223, H2 option	input	ADJ/GND	output	ADJ/GND

The last two rows are the same chip in the same package with different pinouts. The tab is not necessarily ground — on a regulator it is often the output — and the copper under it may be live.

In SOT-89 the middle lead continues into the pad under the body. AZ1117C in SOT-89 also comes in two versions: R is 1 ADJ/GND, 2 output, 3 input; R2 is 1 ADJ/GND, 2 input, 3 output.

Testing with a multimeter

In diode-test mode the meter shows the forward voltage of a junction.

Bipolar transistor

On an NPN, red probe on the base, black on emitter, then collector: roughly 0.5–0.8 V each, open the other way. Collector to emitter is open both ways. A PNP is the mirror image. This tells you both the type and the base. Digital (pre-biased) transistors with built-in resistors read differently; that alone does not mean the part is bad.

MOSFET

• The gate shows no conduction to source or drain.
• Source to drain there is a body diode: on an N-channel part, red on source and black on drain reads about 0.4–0.8 V, the reverse is open.
• To switch it on, touch red to the gate and black to the source; the gate charges and drain–source now reads almost a short. Short gate to source with tweezers and it turns off. This works when the meter’s test voltage exceeds the gate threshold; the BSS138 datasheets give 0.5–1.5 V.

Gates are sensitive to static, so handle the part with tweezers. In-circuit readings are distorted by parallel parts; for a definite answer, remove the part with hot air first.

Common mistakes

• Replacing by code without a datasheet. At Diodes, K1R is three different transistors, and another maker may use the same code for something unrelated.
• Reading the date as part of the code. onsemi marks BC846A as 1A and MMBT3904 as 1AM, each followed by a date letter.
• Assuming the pinout. Base–emitter–collector is not guaranteed; a MOSFET in the same package is gate–source–drain, and other layouts exist.
• Guessing the package. SC-70 and SOT-23 parts with the same code differ in size and power rating.
• Treating the SOT-223 tab as ground. On a regulator it is often the output.
• Wrong gain group. BC847A instead of BC847C has several times less hFE.

In short

• Codes on SOT-23, SC-70, SOT-89 and SOT-223 are not standardised; each manufacturer sets its own.
• Search by code + package + lead count, then confirm in the candidate’s datasheet.
• The last character is often a date or factory mark.
• SOT-23 numbering is fixed (1 and 2 bottom, 3 top); pin functions are not.
• In the examples checked, the SOT-223 tab is tied to pin 2 — which is not necessarily ground.
• Test: bipolar junctions read 0.5–0.8 V; a MOSFET shows a drain–source body diode and turns on when the gate is charged.

Compare with resistors, where the code is standard: SMD resistor marking. In ServiceCenterSoft a part from stock is added as a line in the repair estimate, and stock is kept as a log with quantities on hand and a minimum level, so items running low show up in their own filter. More in spare parts accounting.