BSS138 N-Channel MOSFET – Specifications, Pinout, and Circuit

Introduction

BSS138 is a small N-channel MOSFET used for low-voltage switching and logic level shifting, especially between 3.3V and 5V systems.

Specifications

Type: N-Channel Enhancement Mode
Package: SOT-23
Applications: General-purpose switching, level shifting

Maximum Ratings (at 25°C):

• VDS (Drain-Source Voltage): 50 V

• VGS (Gate-Source Voltage): ±20 V

• ID (Continuous Drain Current): 200 mA

• IDM (Pulsed Drain Current): 800 mA

• PD (Power Dissipation): 310 mW

• TJ, Tstg (Operating/Storage Temp): -55°C to +150°C

Thermal Resistance:

• RθJA: 400 °C/W

Electrical Characteristics (Typical at 25°C):

• VGS(th) (Gate Threshold Voltage): 0.8–1.5 V

• RDS(on) at VGS = 4.5 V: ~3.5 Ω

• RDS(on) at VGS = 10 V: ~2.5 Ω

• IDSS (Off-State Leakage): ≤1 µA

• Ciss (Input Capacitance): ~50–75 pF

BSS138 MOSFET Pinout

The BSS138 has three pins: Gate, Source, and Drain, each serving a specific switching function.

Pin Configuration

• Gate (Pin 1): Input control pin. Apply 2–10V to turn the MOSFET on. Works with 3.3V or 5V logic.

• Source (Pin 2): Connect to ground in low-side switching.

• Drain (Pin 3): Connect to the load or positive voltage.

BSS138 Level Shifter Circuit Explanation

This circuit uses a BSS138 N-channel MOSFET to perform bidirectional logic level shifting between a low-voltage (LV) system (e.g., 3.3V) and a high-voltage (HV) system (e.g., 5V).

Pin Connections:

• Gate is connected to the LV power supply (e.g., 3.3V).

• Source is connected to the LV data line (LV1).

• Drain is connected to the HV data line (HV1).

• Both sides use 10kΩ pull-up resistors to their respective supply voltages.

Working Principle:

• When LV1 is high, the gate-source voltage (Vgs) is near 0V → MOSFET is off → HV1 pulled high by resistor.

• When LV1 is low, Vgs becomes positive (e.g., 3.3V) → MOSFET turns on → HV1 is pulled low through the MOSFET.

• When HV1 is driven low, it also pulls LV1 low via the MOSFET body diode, which then fully turns on the MOSFET and allows proper low-level transfer.

Usage Notes:

• LV must be lower than HV for proper operation.

• Suitable for I²C, UART, and general-purpose bidirectional data lines.

• Keep the MOSFET’s gate tied to LV to protect it from overvoltage.

Conclusion

The BSS138 is a simple, reliable, and widely used MOSFET for level shifting and low-power switching. With proper voltage levels and pull-up resistors, it can be effectively used in I²C, UART, and general-purpose interfaces between 3.3V and 5V systems.