BSS138 Transistor: Features, Circuit, Datasheet and Alternative

The BSS138 is a popular N-channel enhancement-mode MOSFET widely used in electronics for low-voltage switching and signal-level shifting applications. Its compact size, fast switching speed, and compatibility with logic-level voltages make it an essential component in various digital and analog circuits.

What is BSS138?

The BSS138 is an N-channel MOSFET designed primarily for low-voltage, low-current applications. It is commonly used in digital circuits for switching and level-shifting tasks. With a low threshold voltage (V_GS(th)), it can be fully turned on with standard logic-level signals (e.g., 3.3V or 5V), making it ideal for microcontroller-based projects.

BSS138 Features

· N-Channel MOSFET

· Low gate threshold voltage (typically 1.0V to 2.5V)

· Drain current (I_D) up to 200mA

· Drain-Source voltage (V_DS) up to 50V

· Low on-resistance (R_DS(on)) for efficient switching

· Fast switching speed

· Compact SOT-23 surface-mount package

· Logic level compatible (3.3V/5V control)

BSS138 Application

The BSS138 is used in a variety of low-power and logic-compatible applications, including:

· Logic level shifters (e.g., 5V to 3.3V)

· Low-voltage DC switching

· Load switching for microcontrollers

· Signal interfacing between ICs

· I2C and SPI level translation

· Low-side switching of LEDs or relays

BSS138 Datasheet

Key parameters from the BSS138 datasheet:

For full electrical characteristics and temperature ratings, consult the official manufacturer datasheet here.

BSS138 Alternative

If BSS138 is unavailable, consider these equivalent alternatives:

· IRLML6344 – Better R_DS(on) performance

· 2N7002 – Similar characteristics and SOT-23 package

· AO3400A – Lower on-resistance, higher current capability

· IRLML6402 – P-Channel complementary option

Always compare datasheet specs to ensure compatibility with your application.

BSS138 Pinout

The BSS138 comes in a 3-pin SOT-23 package:

    1 – Gate (G)

    2 – Source (S)

    3 – Drain (D)

Always verify pin orientation with the package datasheet when designing PCB footprints.

BSS138 Level Shifter Circuit

A common use case of the BSS138 is as a bidirectional logic level shifter, especially for I2C communication between 5V and 3.3V devices:

Example: 5V ↔ 3.3V I2C Level Shifter

The MOSFET acts as a bidirectional switch. When the 5V side pulls low, the MOSFET conducts and pulls the 3.3V side low. When both sides are high, the MOSFET stays off. This technique allows safe two-way communication across voltage domains.

Conclusion:
The BSS138 is a versatile, logic-level N-MOSFET that remains a staple in electronics design, especially for level shifting and low-power switching. Its simplicity, availability, and proven performance make it an excellent choice for embedded and DIY applications.