BSS138N

BSS138N is a small-signal N‑channel enhancement MOSFET in a SOT‑23 package. It's widely used as a low‑side switch and for bidirectional I2C level shifting.
Key points:
- Type: N‑channel MOSFET (logic‑level gate).
- Typical specs: Vds ~50 V; Id ~0.1–0.18 A; Rds(on) a few ohms at Vgs ≈ 2.5–4.5 V; Vgs(th) ~0.6–1.5 V. Gate‑source max commonly around ±8 V (check the specific datasheet).
- Package: SOT‑23.
- Common uses: microcontroller IO switching, small‑signal switches, and I2C level shifters (often paired with pull‑ups to different voltages).
- How it works: Conducts when Vgs exceeds threshold; channel resistance connects D and S; includes an intrinsic body diode from source to drain.
- Tips: ensure Vds and Id meet your load, keep Vgs within max, and for I2C level shifting use the standard two‑pull‑up arrangement with the gate tied to the lower‑voltage side.
Pros: cheap, compact, readily available. Cons: limited current and higher Rds(on) than larger MOSFETs.

Direct equivalents (N‑channel logic‑level MOSFETs in SOT‑23):
- 2N7002 (the most common substitute)
- Si2302 / Si2302A
- IRLML2502
- IRLML6344
In general, any low-threshold, 20–30 V N‑channel MOSFET in SOT‑23 with similar Id and Rdson can replace BSS138N (suitable for level shifting like I2C).

Key features of BSS138N (N‑channel MOSFET, SOT‑23):
- N‑channel enhancement MOSFET in a small SOT‑23 package.
- VDS: 50 V. ID: ≈ 180 mA (typical continuous drain current).
- RDS(on): ~3 Ω (typical) at VGS = 4.5 V; higher (≈6 Ω) at VGS = 2.5 V.
- Gate threshold: ~0.8–1.5 V (logic‑level device).
- Logic‑level gate drive: usable with 2.5–3.3 V logic, good for level‑shifters (e.g., I2C).
- Low gate charge and input capacitance for fast switching.
- Wide operating temperature range (typical: −55 °C to +125 °C).
- RoHS compliant, suitable for discrete switching and general purpose tasks.
- Common applications: level shifting, load switching, small‑signal switching, compact DC‑DС tasks.
Note: exact figures can vary by vendor; refer to the specific datasheet for precise numbers.

- Pin count: 3 pins
- Function: N-channel enhancement-mode MOSFET (logic-level) in a SOT-23 package, used as a small-signal switch or level-shift device; gate controls conduction between drain and source (with a body diode from drain to source).

- Manufacturer: Fairchild Semiconductor (now part of ON Semiconductor).
- What kind of company: A US-based semiconductor manufacturer that designs and produces discrete devices (like transistors and MOSFETs) and integrated circuits; it was acquired by ON Semiconductor in 2016.

BSS138N is a small N‑channel enhancement MOSFET. Common application areas:
- Bidirectional level shifting for open-drain buses (I2C) between different logic levels (e.g., 1.8V/3.3V/5V)
- Level translation for other open-drain interfaces (1-Wire, SMBus)
- Low-side switch for small-load applications in portable electronics (LEDs, sensors, microcontroller peripherals)
- General-purpose signal switching in compact PCB designs where a gate can be driven from a low voltage
Notes: suitable for low-current, space-constrained applications; check Vds, Id, and Rds(on) specs for your load.

BSS138N is an N-channel MOSFET in a SOT-23 (3-pin) surface-mount package.