BSS138 — N-Channel Logic-Level Enhancement Mode MOSFET (SOT-23)

Overview

The BSS138 is a low-threshold N-channel Enhancement Mode MOSFET IC manufactured by ON Semiconductor, Fairchild, and Diodes Inc. Packaged in a 3-pin surface-mount SOT-23 enclosure, it is famous as the core switching element behind bidirectional $3.3\text{V} \leftrightarrow 5.0\text{V}$ $I^2C$ logic level converter modules (such as Adafruit, SparkFun, and generic AliExpress 4-channel level shifter boards).

Offering a maximum drain-source breakdown voltage of $50\text{ Volts}$ , continuous drain current of $220\text{ mA}$ , and a low gate threshold voltage ( $V_{GS(th)}$ of $0.8\text{V} \dots 1.5\text{V}$ ), the BSS138 turns on fully at low gate voltages ( $2.8\text{V} \dots 3.3\text{V}$ ), making it ideal for low-voltage sensor interfacing.

Quick reference


Transistor Type	N-Channel Logic-Level Enhancement Mode MOSFET
Package	SOT-23 (3-pin SMD)
Drain-Source Voltage ( $V_{DSS}$ )	$50\text{ V}$ max
Continuous Drain Current ( $I_D$ )	$220\text{ mA}$ continuous ( $880\text{ mA}$ pulsed)
Gate-Source Voltage ( $V_{GS}$ )	$\pm 20\text{ V}$ max
Gate Threshold Voltage ( $V_{GS(th)}$ )	$0.8\text{ V}$ min to $1.5\text{ V}$ max ( $1.3\text{ V}$ typical)
On-State Resistance ( $R_{DS(on)}$ )	$3.5\ \Omega$ at $V_{GS} = 10\text{V}$ / $6.0\ \Omega$ at $V_{GS} = 4.5\text{V}$
Turn-on / Turn-off Speed	$2.5\text{ ns}$ turn-on / $9\text{ ns}$ turn-off (Ultra fast)

Pinout (SOT-23 Package)

Looking at the top of the SOT-23 surface-mount component with single lead on top:

text

               ┌─────────┐
               │    3    │  (DRAIN)
               └─┐     ┌─┘
                 │BSS138│
               ┌─┘     └─┐
               │ 1     2 │
               └─────────┘
            (GATE)   (SOURCE)

Pin	Name	Description
1	`GATE` (`G`)	Gate control input (Connect to 3.3V power rail in level shifters)
2	`SOURCE` (`S`)	Source terminal (Low-voltage 3.3V bus side)
3	`DRAIN` (`D`)	Drain terminal (High-voltage 5.0V bus side)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Drain-Source Voltage	$V_{DSS}$	50	—	—	V	$V_{GS} = 0\text{V}, I_D = 100\ \mu\text{A}$
Gate Threshold Voltage	$V_{GS(th)}$	0.8	1.3	1.5	V	$V_{DS} = V_{GS}, I_D = 1\text{mA}$
On-Resistance ( $V_{GS}=10\text{V}$ )	$R_{DS(on)}$	—	1.4	3.5	Ω	$V_{GS} = 10\text{V}, I_D = 220\text{mA}$
On-Resistance ( $V_{GS}=4.5\text{V}$ )	$R_{DS(on)}$	—	1.8	6.0	Ω	$V_{GS} = 4.5\text{V}, I_D = 220\text{mA}$
Input Capacitance	$C_{iss}$	—	27	50	pF	$V_{DS} = 25\text{V}, f = 1\text{MHz}$

Bidirectional $I^2C$ Level Shifter Circuit (1 Channel)

text

        +3.3V Power Rail                    +5.0V Power Rail
              │                                   │
      [ R1 = 10kΩ Pullup ]               [ R2 = 10kΩ Pullup ]
              │                                   │
  3.3V MCU ───┴─── [Pin 2: SOURCE] ────┐          ├─── 5V Sensor (SDA/SCL)
                        BSS138         │          │
                   [Pin 1: GATE] ──────┼── +3.3V  │
                                       │          │
                   [Pin 3: DRAIN] ─────┴──────────┘

Common mistakes

Using BSS138 for high-power DC load switching: BSS138 has an $R_{DS(on)}$ up to $6.0\ \Omega$ . Passing more than $200\text{ mA}$ results in substantial thermal dissipation in the SOT-23 package. Use a low-resistance MOSFET like the AO3400 for driving motors or LEDs.
Forgetting pull-up resistors in level shifter circuits: The BSS138 level shifter circuit relies on external pull-up resistors (typically $10\text{ k}\Omega$ ) on both the 3.3V and 5V sides. Without pull-up resistors, lines cannot be pulled HIGH.

Notes

BSS138 vs 2N7002: BSS138 has a lower $V_{GS(th)}$ max ( $1.5\text{V}$ vs $2.5\text{V}$ ) and faster switching speed ( $2.5\text{ ns}$ vs $7\text{ ns}$ ), making it the preferred choice over 2N7002 for $3.3\text{V}$ $I^2C$ level shifter circuits.