BSS138 MOSFET — Specifications & Equivalents

About the BSS138

The BSS138 is an N-channel MOSFET with a drain–source voltage (V_DS) rating of 50 V, a continuous drain current (I_D) of 0.22 A and an on-resistance (R_DS(on)) of 3.5 Ω. It can dissipate up to 0.36 W and is rated to a junction temperature of 150 °C. It is supplied in the SOT23 package.

At a glance

• Type: N-channel MOSFET
• Max drain–source voltage (V_DS): 50 V
• Max continuous drain current (I_D): 0.22 A
• On-resistance (R_DS(on)): 3.5 Ω
• Power dissipation (P_D): 0.36 W
• Max junction temperature (T_J): 150 °C
• Package: SOT23

Typical applications

With its 50 V drain–source rating the BSS138 sits in the low-voltage class of switching MOSFETs, typically found in 12 V and 24 V automotive and industrial rails, synchronous buck converters, load switching and reverse-polarity protection and H-bridges for small motors.

At 0.22 A it is aimed at small-signal and light-load switching rather than power conversion.

On-resistance of 3.5 Ω is relatively high, so this part is better suited to low duty-cycle or low-current switching than to continuous high-current conduction.

Gate drive and switching notes

Keep gate–source voltage within the ±20 V maximum; exceeding it punctures the gate oxide and destroys the device permanently.

Choosing a replacement for the BSS138

A drop-in substitute for the BSS138 must be the same N-channel type and offer a drain–source rating of at least 50 V, a continuous drain current rating of at least 0.22 A and an on-resistance no higher than 3.5 Ω if efficiency and heat matter.

Mechanically, check the replacement against the SOT23 footprint and confirm the pin order — parts sharing a package outline do not always share a pinout.

Where the application switches inductive loads or runs near the voltage limit, keep extra headroom rather than choosing a part that only just meets the original numbers.

Frequently asked questions

What is the BSS138?

The BSS138 is an N-channel MOSFET (metal-oxide-semiconductor field-effect transistor). It is rated for 50 V drain-source and 0.22 A continuous drain current.

What is the maximum voltage of the BSS138?

The BSS138 has a maximum drain-source voltage (Vds) of 50 V. Treat this as an absolute maximum and design with margin, especially when switching inductive loads.

How much current can the BSS138 handle?

It is rated for 0.22 A of continuous drain current, with power dissipation limited to 0.36 W. Real-world capability depends on your heatsinking and ambient temperature.

What is the on-resistance of the BSS138?

On-resistance (Rds(on)) is 3.5 Ω. Multiply by the square of your load current to estimate conduction loss.

What package does the BSS138 come in?

It is listed in SOT23. Verify the pinout of the specific variant, as one outline can carry different pin assignments.

What can I use to replace the BSS138?

No verified equivalents are listed yet. Choose a part of the same polarity that meets or exceeds every rating above and shares a compatible package and pinout.