What Is a SOT-23 Bipolar Transistor?

A SOT-23 bipolar transistor fixes footprint, pinout, thermal path and marking. How the outline sets usable current, and when to move to a smaller package.

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Updated: October 28, 2026

SOT-23 bipolar transistors put a small BJT die in a three-pin surface-mount outline common on dense boards. The package is a mechanical standard—pinout and ratings still come from the specific datasheet.

Key Takeaways

The outline fixes the pinout. In the SOT-23 small-signal transistor families the three leads are base, emitter and collector in a defined order, and the same die is offered in the smaller SOT-323 and SOT-416 outlines.
Thermal performance is part of the package promise: 250 mW of total dissipation at a 25 °C ambient is quoted for FR4 board with a standard footprint, not for any board.
Marking codes are short and cryptic by design. A part you cannot identify after the reel is gone is a part you cannot verify at rework.
The base-emitter voltage rating is 6 V with the collector open — lower than most designers expect, and easy to exceed with a poorly referenced base.
Moving to a smaller outline trades board area for thermal headroom; moving up buys dissipation at the cost of space.

What the Package Fixes, and What It Leaves Open

The package determines the footprint, the pin order and the thermal path; the datasheet determines everything else.

The Nexperia PMBT3904 and MMBT3904 devices are a 40 V, 200 mA NPN switching transistor in SOT23 — a plastic surface-mounted package with three terminals on a 1.9 mm pitch and a 2.9 × 1.3 × 1 mm body, with the pins assigned base, emitter and collector and a two-character marking code. The electrical ratings sit alongside: V_CBO 60 V, V_CEO 40 V, V_EBO 6 V, collector current 200 mA, DC current gain 100 to 300 at V_CE = 1 V and I_C = 10 mA, 250 mW dissipation at a 25 °C ambient on an FR4 board with a standard footprint, a 150 °C junction limit, and AEC-Q101 qualification.

Package factor	What SOT-23 provides	Why it matters in a design review
Outline and pitch	Three leads, 1.9 mm pitch, 2.9 × 1.3 × 1 mm body	Footprint, placement density and stencil design
Pin assignment	Base, emitter, collector in a defined order	A mirrored footprint is a dead board, not a schematic error
Thermal path	250 mW at a 25 °C ambient, FR4 with standard footprint	The derating basis — less copper or a warmer enclosure means less power
Identification	Two-character marking code	Rework and incoming inspection depend on it
Smaller alternatives	The digital transistor family offers the same function in SOT-323, SOT-416 and a leadless SOT-883 body	Density can be traded against thermal headroom deliberately
Qualification	AEC-Q101 qualified	Automotive programmes can use the same package

Choosing Between the Outlines

The decision is usually thermal, and it is arithmetic rather than instinct.

Take the same 100 mA load used for base-drive sizing in this class: with a saturation voltage of roughly 0.2 V, the transistor dissipates about 20 mW — a small fraction of the 250 mW rating, and the junction rise on a standard SOT-23 footprint stays modest. That is the case where moving to a smaller outline costs nothing. The reverse case is a transistor that has to pass 200 mA continuously: the dissipation rises towards 40 mW and the derating from a 25 °C ambient becomes the governing constraint, which is when the larger body, more copper, or a different device class is the answer.

The boundary conditions are the same ones that apply to any three-lead small-signal part. The 250 mW figure assumes a particular board and ambient, so it is the starting point of a derating calculation rather than an operating point. The 6 V emitter-base rating catches designs that pull the base below the emitter — for example when a driver output is high while the transistor’s emitter is pulled low by the load. And the marking code has to be recorded with the design, because a substitution without it leaves the board unverifiable.

Pinout is not universal

Many SOT-23 BJTs use a BCE or CBE order—but not all. Always map pins from the PDF for that MPN. Competitors’ distributor pages show how often reverse-engineered boards fail from assuming 2N3904 pinout in a different SOT-23 part.

Thermal and pre-biased options

SOT-23 dissipates little power without copper. Derate collector current with temperature. Digital/pre-biased (resistor-equipped) SOT-23 BJTs integrate base resistors for logic interfacing—useful, but check the internal divider before treating them as bare transistors.

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