BSS138 MOSFET : Principle，Features & Applications

For many years, we have known various 3.3V devices such as the ESP8266, ESP32, HC-05 Bluetooth module, Nokia 5110 LCD, and BMP180 barometric pressure sensor. To connect a 3.3V SPI or I2C device to a 5V device like an Arduino, which supports bi-directional communication, a logic level shifter or an external level shifter is used. The logic level shifter utilizes one BSS138 MOSFET per bus to enable bidirectional level shifting and protect the low voltage side from the high voltage side. This article provides a brief description of the N-channel logic level MOSFET known as the BSS138.

Alternatives to the BSS138 MOSFET include the 2N7000, 2N7002, NTR4003, FDC558, FDC666, and BS170. Equivalent N-channel MOSFETs to the BSS138 are the IRF3205, IRF540N, IRF1010E, 2N7000, BS170N, FDN358P (P-channel MOSFET), and BSS84 (P-channel MOSFET).

What is BSS138 MOSFET?

The BSS138 is a low resistance (3.5 ohms) and low input capacitance (40 pF) logic-level N-channel MOSFET in the SMD package. These MOSFETs can switch at a high speed of 20 ns. Because of their higher switching speed and low threshold voltage, they are predominantly used in level shifting circuits.

This compact N-channel logic level MOSFET has a very low threshold voltage of 0.5 Volts, making it suitable for all low voltage and level shift applications. It also features low input capacitance and low on-resistance, making it efficient for switching circuits. Due to its compactness, the BSS138 MOSFET is commonly used in portable devices such as cell phones and other power management circuits.

The BSS138 is a bit pricey for its specifications, so for a cheaper alternative, check out the 2N7002. However, some adjustments are required for the MOSFET’s on-state resistance and threshold voltage. These MOSFETs are available in an SMD package, making them suitable for smaller applications. The main disadvantage of this MOSFET is its low drain current. It can provide 200mA continuous current and up to 1A peak current at maximum threshold voltage. If exceeded, the MOSFET would get damaged.

BSS138 MOSFET Pin Configuration/Pin Diagram:

The BSS138 is a 3-terminal or 3-pin N-channel logic-level MOSFET. The BSS138 pin configuration/pin diagram is shown in the figure below.

• Pin 1: Source: The current flows out through this terminal.
• Pin 2: Gate: The MOSFET biasing is controlled by this terminal.
• Pin 3: Drain: The current passes in through this terminal.

BSS138 MOSFET Specifications:

The BSS138 specifications and characteristics are listed below.

• It is an N-channel MOSFET logic level with low on-state resistance.
• The continuous drain current ID is 200mA.
• Drain-Source voltage VDS is 50V.
• The minimum gate threshold voltage VGS is 0.5V.
• The maximum gate threshold voltage VGS is 1.5V.
• On-State resistance is 3.5 ohms.
• Turn On and Turn Off time is 20ns each.
• Available in SOT23 SMD package.

The absolute electrical and thermal characteristics at T=25°C:

• Drain-Source voltage VDSS: 50V.
• Gate-Source voltage VGSS: ±20V.
• Continuous drain current ID: 0.22A.
• Pulsed drain current: 0.88A.
• Maximum power dissipation: 300mW.
• Operating and storage junction temperature range: -55°C to +150°C.
• Maximum lead temperature for soldering purposes: 300°C.
• Thermal resistance: 350°C/W.
• Input capacitance @ VDS=25V, VGS=0V, f=1MHz: 27pF.
• Output capacitance @ VDS=25V, VGS=0V, f=1MHz: 13pF.
• Reverse transfer capacitance @ VDS=25V, VGS=0V, f=1MHz: 6pF.
• Gate resistance: 9 ohms.

Circuit Diagram/How to Use?

Now, let’s understand the circuit diagram and how to use the N-channel BSS138 logic level MOSFET as a bi-directional level shifter. Consider the circuit diagram below, which includes a BSS138 N-channel MOSFET with an internal drain-substrate diode (which must be present). Divide the circuit into two parts: the left, or low voltage side, and the right, or high voltage side.

Both sides have different supply voltages and logic levels. Pull the bus on the low voltage side HIGH to 3.3 Volts, with the device powered by a 3.3 Volt supply on this side. The bus on the right side is pulled HIGH to 5V, with the device powered by a 5V supply. The MOSFET’s gate terminal must be connected to the low voltage supply of 3.3V. The source and drain terminals of the MOSFET are connected to the low voltage bus and the high voltage bus, respectively. This simple circuit functions as a bidirectional logic level shifter.

Now, let’s examine how the above circuit works as a bi-directional logic level converter. To get a clearer understanding of its operation, let’s divide its functioning into three states.

Standby State

Firstly, when there is no signal from either side, it will generate either a high output signal or take it as input. In practice, this means that there is nothing to lower the signal level, so we need to pull the output to logic high (for example 3.3V for Raspberry Pi, 5V for Arduino) at both ends.

The gate-to-source junction voltage VGS (pin 1 and pin 2) is 0V (both 3.3V), therefore the MOSFET is in the OFF state. Hence, the output on the LV1 side is based on a resistor R1 (10k) which can pull to 3.3V, and the output on the HV1 side is based on R2 which can pull to 5V. So, both ends are at a higher logic level.

3.3Volts Side is Pulled Low

If you want to get the output of low voltage from the low voltage side, then connect the output to 0V through an open drain. This pulls the Q1 source to 0V and 3.3V at the gate terminal, which makes the MOSFET turn ON. Then the high output (at the gate terminal) from Q1 goes low. Thus, this results in a logic low-level output on the high voltage side.

5 Volts Side is Pulled Low

Now let’s understand how to pull the bus low to handle the transition from a high level on both sides to the 5V side. The higher side now has a lower output by pulling the end down through the open drain. The MOSFET’s drain-substrate diode substrate is pulled down (as seen in the MOSFET schematic) so that it can flow from the source to the drain (opposite to the conducting current, when it works as a switch).

Then, when the voltage of the source drops below the voltage of the gate, the MOSFET turns ON. When the MOSFET is in an ON state, both LV and HV sides are at 0V, providing a logic low level on each side.

When these three states are combined, the logic levels can be shifted in both directions, and hence it works as a bidirectional logic level shifter.

Applications of BSS138/Where to Use?

The following are the applications of BSS138.

• Used in low voltage and low current applications.
• Used in bidirectional logic level shifter circuits.
• Used in DC-DC converters.
• Used in applications where low on-state resistance is needed.
• Used in e-mobility applications.
• Used in power management applications.
• Please refer to this link to know more about the BSS138 MOSFET Datasheet.

Thus, this is all about an overview of the BSS138 N-channel logic-level MOSFET. These MOSFETs are designed to minimize on-state resistance and provide fast switching performance. They are more reliable and rugged, making them suitable for low current and low voltage applications such as servo motor control, power drive circuits, and other switching applications.