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)

Provenance

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

Datasheet: Diodes Incorporated datasheet (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