# BSS138

**Diodes Incorporated** · N-channel logic-level MOSFET · SOT-23

A small N-channel logic-level MOSFET in SOT-23, most famous as the switch in bidirectional I2C and logic level shifters. Its gate threshold is low enough (around 1.5 V) to turn fully on from 3.3 V or even lower logic, and it handles up to 50 V drain-source and a couple hundred milliamps, so it also drives small loads, LEDs, and gate signals. In the classic level-shifter circuit one BSS138 per line, with pull-ups to each rail, passes signals both ways between a 3.3 V and a 5 V bus. Pinout is G, S, D.

## Pinout

| Pin | Name | Type | Function |
|---|---|---|---|
| 1 | G | input | Gate. Controls the MOSFET channel. VGS(TH) typ 1.2 V (0.5 V min, 1.5 V max). Continuous VGSS = ±20 V max. Input capacitance Ciss ≤ 50 pF @ VDS = 10 V. |
| 2 | S | passive | Source. Reference terminal for VGS; connects to the internal body-diode anode. Continuous drain current ID = 200 mA (IDM pulsed = 1 A). |
| 3 | D | passive | Drain. Switched terminal; internal body-diode cathode. BVDSS = 50 V min, RDS(ON) typ 1.4 Ω (3.5 Ω max) @ VGS = 10 V, ID = 0.22 A. |

## Key specifications

| Parameter | Value |
|---|---|
| Type | N-channel MOSFET (logic-level) |
| Drain-source (VDS) | 50 V |
| Gate threshold (Vgs(th)) | 0.8-1.5 V |
| Continuous drain current | ~200 mA |
| RDS(on) | ~3.5 Ω at VGS=10 V |
| Common use | I2C / logic level shifting |
| Pinout | G-S-D |
| Package | SOT-23 |

## Ordering variants

Same part, different packaging or reel option.

- `BSS138-7-F (SOT-23)`
- `BSS138W-7-F (SOT-323)`

## CAD (KiCad official)

- Symbol: `Transistor_FET:BSS138`
- Footprint: `Package_TO_SOT_SMD:SOT-23`
- 3D model: SOT-23 (KiCad packages3D)
- 3D model files: `bss138-nmos-etched.step` / `bss138-nmos-etched.glb` carry the MPN laser-etched on the die in a toggleable `Adom.LaserEtch` group (Fusion-ready); `bss138-nmos.step` / `bss138-nmos.glb` are the bare package body.

## Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

- **Datasheet:** [Diodes Incorporated datasheet](https://www.diodes.com/assets/Datasheets/BSS138.pdf) (6 pp, sha256 `c93dcb33f41424e6`, retrieved 2026-08-17, tier: manufacturer)
- **Extractor:** `ds2sf` (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
- **CAD:** KiCad official libraries (CC-BY-SA 4.0).

[Datasheet PDF](docs/bss138-nmos-datasheet.pdf)