BSS138 SOT-23 MOSFETs and Related SMD Transistors
BSS138 MOSFETs for board-level design, not finished level-shifter boards
This collection contains bare BSS138 surface-mount N-channel MOSFETs and multi-model SOT-23 semiconductor listings. BSS138 is commonly discussed as a building block in a bidirectional open-drain logic-level translation circuit, but the actual items here are individual SMD components in selected packs, not assembled level-shifter modules. A BSS138 transistor by itself has no connector, independent low- and high-voltage rails, pull-up network or finished PCB. Buyers designing an I²C translator must supply and validate the complete circuit; buyers seeking a ready-to-use breakout should not assume one is included with a loose-chip order.
What a BSS138 variant supplies
Selected products describe BSS138 or “BSS138 J1/J1Y” in a small SOT-23 surface-mount package. Some are explicitly listed as 50PCS BSS138, others as 100PCS BSS138, and the multi-model listings include a separate “100PCS BSS138” option. These are packaging counts of selected transistors, not numbers of assembled channels. One BSS138 listing's title mentions 0.2 A and 50 V; do not apply those figures to every selected FET, to a finished circuit or to continuous operation without checking the specific manufacturer's datasheet and temperature-dependent operating limits. Gate threshold voltage is also not the same as guaranteed low-resistance operation at a given logic drive voltage.
BSS138 is an N-channel MOSFET; in common open-drain translator topologies a separate appropriate MOSFET is used for each translated signal line, together with correctly chosen pull-ups and rails. A chip pack does not provide a four-channel or eight-channel converter simply because the part is often used in such designs. Confirm drain, gate and source orientation against the actual marking, package drawing and land pattern, rather than relying on a generic SOT-23 photo.
The selected model matters more than the parent title
Several listings place BSS138 alongside 2N7002 and 2N7002K, BSS123, AO3400/AO3402/AO3404/AO3416, AO3401/AO3407/AO3409/AO3413/AO3415, BSS84, 8205A/8205S and CJ-series choices such as CJ2302, CJ2304, CJ2306, CJ2310, CJ2312, CJ3134K, CJ3400 and CJ3402. In those products the selected option determines which component is supplied. “100PCS AO3401” is not 100 BSS138 parts; “10PCS 8205A” does not deliver a hundred-device assortment. P-channel selections such as BSS84 or certain AO34xx types have a different channel polarity and are not interchangeable with an N-channel BSS138. A similar package outline or a “MOSFET” category tag does not establish the same pinout, voltage rating, gate-drive condition or switching performance.
Another listing offers variants marked BAW56, BAV70, BAS16, BAV99 and 1SS226 as well as BSS138, 2N7002, 2SK3018 and TL431. The BAV/BAW/BAS/1SS choices are diode parts and TL431 is an adjustable reference/regulator-type component, not BSS138 MOSFETs. A selected “BAV70 A4 100pcs” or “TL431 SOT-23 100pcs” must be treated as its own device type despite the broad parent title. Printing codes such as J1 are not unique enough by themselves to prove device identity across manufacturers and packages. Verify the full part number and supplier-specific marking convention before replacing a design component.
Where BSS138 level shifting may fit
A properly engineered MOSFET-and-pull-up topology can be useful on an open-drain interface, including certain I²C buses connecting different logic-voltage domains. Whether it meets a particular bus speed depends on the actual MOSFET, rail voltages, pull-up resistors, line capacitance, rise time and connected devices. Each side's pull-up must respect the input voltage ratings of its attached electronics. Power-up, shutdown and partial-power behavior also require review. This collection does not claim that a loose BSS138 pack, by itself, “supports 3.3 V/5 V I²C” in every assembly or guarantees any operating frequency.
A push-pull UART, SPI clock or other actively driven line is not automatically suited to the same passive bidirectional topology. Direction, duty cycle, output impedance and edge-rate requirements are different from an open-drain bus. For those signals, evaluate a dedicated direction-controlled or purpose-designed translator and its load conditions rather than extrapolating from an I²C application note. The BSS138 is not an RS-232 or RS-485 line driver, an isolated interface, a USB bridge or a high-current power converter.
Electrical and mechanical checks before PCB design
Consult the exact selected device's datasheet for drain-source maximum voltage, gate-source maximum voltage, specified on-resistance at the actual gate drive, leakage, capacitances, current limits, thermal resistance and safe operating conditions. Absolute-maximum numbers do not mean a design should routinely operate at those boundaries. A small SOT-23 package's heat dissipation depends strongly on PCB copper area and ambient temperature. For switching loads, review inductive transients and protection separately; a product title using “switch MOS transistor” does not imply integrated protection.
Inspect SOT-23 footprint, pin-number orientation, solder process and whether parts arrive loose or in tape. The same housing style can contain a transistor, a diode pair or a voltage reference. Part marking and selected option must match the board's bill of materials. Supplier text that labels a MOSFET as a “voltage regulator” is a catalog classification, not proof that the component can regulate a supply. Nor does a “high quality” description establish tested parameters, certification, original manufacturer status or cross-vendor equivalence.
How to read quantity options
Multi-model parent listings generally provide separately selectable 100PCS packs for BSS138, 2N7002 and many AO/CJ choices. The 8205A and 8205S options in several listings specify 10PCS instead; do not infer a uniform quantity for the entire product. A 50pcs CJ-series listing offers one chosen CJ number or BSS138 in a 50-piece pack. A 100PCS SOT-23 listing with diode, reference and transistor options likewise offers the selected device only, not all nine model names. “China Mainland” in the variant label is a location attribute, not a second component in the pack. If the product has a single Default Title variant, use its own title and packing statement to determine whether the BSS138 pack is 50 or 100 pieces.
Engineering purchase checklist
- Choose the exact part number and channel polarity. Confirm BSS138 instead of 2N7002, BSS123, BSS84, AO/CJ variants or a non-MOSFET component.
- Verify the selected count, typically 50PCS or 100PCS for BSS138 listings here, and distinguish loose SMD devices from a finished translation board.
- For level translation, document both pull-up rails, target I/O tolerances, bus capacitance and timing. Determine the number of MOSFETs and other passive parts required for the actual circuit.
- Validate package marking, SOT-23 pinout and PCB land pattern from the device maker's information. A code printed on a tiny package alone may be ambiguous.
- For switching rather than level shifting, review gate drive, load current, dissipation, transient protection and temperature using the exact selected MOSFET's electrical and thermal ratings.
- Use a dedicated circuit or translator board when the application requires a preassembled multi-channel converter, push-pull high-speed signals or guaranteed interface behavior beyond a validated discrete design.
Frequently asked questions
Is this an assembled BSS138 level-shifter module? The reviewed products list bare SOT-23 semiconductors and selected packs, not assembled converter boards. Check each exact product before ordering.
Does a 100PCS parent listing include every transistor in its title? No. For multi-model products the variant chooses one named model and stated quantity. Choose “100PCS BSS138” if that is the intended device.
Can a single BSS138 translate four I²C lines? A common discrete topology uses a suitable transistor per signal line plus pull-ups and appropriate supply rails. A pack of components is not a prewired multi-channel board.
Are BSS84, 2N7002 and BSS138 interchangeable? No. Their channel type, pinout, ratings and drive characteristics must be compared to the specific circuit; BSS84 is a P-channel selection rather than the same N-channel part.
Can I put a BAV diode or TL431 reference in the BSS138 footprint? Sharing SOT-23 packaging does not give those different devices the same function or pin assignment. Follow the selected part's schematic and datasheet.
Will BSS138 automatically convert every 5 V and 3.3 V signal? No. A validated complete topology may suit certain open-drain signals, while push-pull and fast interfaces need separate analysis or a purpose-matched translator.
Is the 0.2 A / 50 V title a working guarantee? No. It is wording on one product. Confirm the exact selected manufacturer's ratings and derating conditions, especially gate drive and thermal limits.
These listings supply selected electronic components. Successful voltage-level translation depends on the complete circuit, the exact transistor and the connected devices' electrical specifications.
