BSS138K: Overview, Features, and Applications


The BSS138K is a N-Channel Logic Level Enhancement Mode Field Effect Transistor (FET) from ON Semiconductor and other manufacturers. It's designed for high-speed switching applications where power requirements are low.

Here's a brief overview of this transistor, its features, and common applications:


The BSS138K is an SOT-23 packaged N-Channel MOSFET with a drain to source voltage rating of 50V. It uses a small gate charge and has a low on-resistance (Rds(on)).



  • N-Channel Design: This means the conduction (current flow) happens when a negative voltage is applied to the gate with respect to the source terminal.
  • High Voltage Rating: Drain to source voltage (Vds) rating is up to 50V, making it suitable for a range of higher voltage applications.
  • Drain Current: Continuous drain current (Id) rating of up to 200mA, which is suitable for a variety of lower power applications.
  • Logic Level Gate Drive: The BSS138K can be driven by low voltage, making it suitable for interfacing with logic circuits. It will fully turn on (saturate) with a gate-to-source voltage of less than 5V.
  • Low On-Resistance (Rds(on)): Rds(on) at 10V Vgs is typically around 3.5 Ohms, allowing a relatively large current to flow with a small voltage drop, which improves efficiency and reduces heating.
  • Fast Switching: Because of small input and output capacitances, it has a fast switching speed.
  • Industry-Standard SOT-23 package: Ease of integrating it onto a variety of PCB designs.


Given its features, the BSS138K can be employed in a variety of applications:

  • Logic Level Shifting: It is often used for level shifting in circuits that deal with different operating voltages (for example, interfacing 5 V logic with 3.3 V logic).
  • Switching Loads: Used for low power switching applications such as turning on/off LEDs, small motors, relays, etc.
  • Battery Power Management: Often used in battery management and DC energy harvesting applications because of its low on resistance and high efficiency.
  • General Purpose Switching: Used widely in general purpose digital circuits, for tasks such as driving loads, switch-mode power supplies, etc.


As always, it's important to check the exact specifications from the manufacturer's datasheet for full and accurate details. The manufacturer's datasheet will also have information on the Absolute Maximum Ratings for the device which are key to understanding its limitations and ensuring it is operated within its ratings to prevent premature device failure.