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Fundamentals of MOS FET Relays (No Movable Contacts): Basics

What is a MOS FET Relay?

A relay is a component that receives electrical signals from outside sources and turns on/off or switches electrical circuits. It is mainly used when electrical isolation between the input-side and output-side circuits is desired, thereby reducing noise effects and safety risks.

Electrical signal: Input => Relay: Control => Electrical circuit ON/OFF: Output

MOS FET relays are non-contact relays that use MOS FETs (metal oxide semiconductor field effect transistors) to switch control signals.
MOS FETs are sometimes called photocouplers as a type of coupler, but because they are relatively new products, each company has given them various names and trademarks.
The table below shows examples of products equivalent to our MOS FET relay (G3VM).

According to Omron's research: as of December 2023

Manufacturer Catalog name
Toshiba Photorelay
Panasonic PhotoMos relay
Okita Works PhotoDMOS-FET relay
Omron MOS FET relay
Infineon Technologies Solid state relay
BROADCOM Solid-state relay

Note: Other company names and product/service names may be used as trademarks or registered trademarks of their respective companies.

How is a MOS FET Relay Positioned?

Relay technology can be divided into two main categories: Movable contacts (mechanical relay) and no movable contacts (solid state relay). MOS FET relays are among the no movable relays.

Movable contacts (Mechanical Relay)


This type of relay has contacts that are mechanically actuated to open/close by a magnetic force to switch signals, currents and voltages ON or OFF.

No movable contacts (Solid State Relay)



Unlike mechanical relays, this type of relay has no moving contacts but instead employs semiconductor and electrical switching elements. By the operation of these electronic circuits, signals, currents and voltages are switched ON or OFF electronically.

MOS FET relays

Semiconductor relays for printed circuit boards (PCBs) using MOS FETs as output elements

Power MOS FET relays

Semiconductor relays for power switching for control panels and PCBs using phototriacs and phototransistors as output elements

What are the Types of MOS FET Relays?

MOS FET relays are non-contact relays (solid-state relays) and can be classified as a type of photocoupler because they are components consisting of a light emitting diode (LED) and a light receiving element. There are three major types of photocouplers, and the primary difference is the type of light-receiving element used.
MOS FET relays are products that incorporate a PDA (Photodiode Array) and a MOS FET chip in the light receiving element.

Photocoupler: LED (light emitting diode) + Light receiving element

Transistor coupler

LED + Light receiving element (phototransistor)

A transistor that is operated by light. It is unidirectional and can only control direct current. Output characteristics depend on the input current value.


Triac coupler

LED + Light receiving element (phototriac)

A triac that is operated by light. It is bidirectional and can control alternating current. Once triggered, the output current remains ON until it reaches zero, so DC cannot be interrupted.


MOS FET relay

LED + Light receiving element (PDA + MOS FET)

A MOS FET that is operated by light. It can control both direct current and alternating current. Output characteristics do not depend on the input current value. Some models can also switch ampere-level currents with small input currents.


What are the Structure and Principles of MOS FET Relays?

MOS FET relays are semiconductor relays that use MOS FETs as output elements to switch signals. Unlike mechanical relays that use movable contacts to open and close circuits, MOS FET relays are contactless and have advantages over mechanical relays, such as fast response, compact size, long life, and quiet operation.

MOS FET relay structure

① LED chip
Semiconductor device that emits light
Emits light according to the input current
② PDA chip (photodiode array)
Photodiode array (solar cell + control circuit)
Semiconductor device that detects light and generates electricity
Generates electricity according to the amount of light received to supply the gate voltage for the MOS FET chip
③ MOS FET chip
A type of transistor
Driven according to the voltage of PDA chip
Switches signals on electronic circuits

MOS FET Relay Operating Principle

Operating principle and function of MOS FET relay

1.LED, 2.PDA (solar cell), 3.MOS FET
The LED emits light when current is applied to the input side.
The PDA on the output side receives the light and generates electricity, which is then converted back into voltage.
The voltage generated by the PDA goes through the control circuit to become the gate voltage, which drives the MOS FET.

What are the Drive Systems for MOS FET Relays?

MOS FET relays can be controlled in two major ways.

・Current drive system (MOS FET relays in general)
This product can be used at the optimum drive current by designing the limiting resistor according to the drive voltage.
・Voltage drive system (some MOS FET relay series)
This product saves space and eliminates the need for input-side resistor selection by incorporating the limiting resistor required for drive current control.

How are MOS FET Contacted Configured?

As shown in the table below, there are two types of MOS FET relays: SPST-NO (1a) and SPST-NC (1b).

Input current SPST-NO (1a) SPST-NC (1b)
Not energized

If you require SPDT, please consider the G3VM-M series of module type that realizes SPDT contact configuration.

Further evolution through modularization MOS FET Relay Module

How are MOS FET Relays Connected?

MOS FET relays can handle both direct current and alternating current by incorporating two MOS FET chips.
In addition, the 6-pin MOS FET relay type offers the following advantages by changing the connection method of the output-side terminals.

Connection Features Applicable load

Connection A

Can handle both DC and AC


Connection B

On-resistance can be lowered
(Half as connection A)


Connection C

Load current can be increased
(Twice as connection A)


What are Features of MOS FET Relays?

The followings are characteristics that are unique to the function of turning on/off or switching electrical circuits.

  • Ultra-compact & lightweight
  • Long life
  • Stable ON resistance
  • Quiet operation
  • Fast response
  • High withstand voltage, etc.
Security, Measurement & communication, Medical equipment, Industrial equipment, Office automation equipment, Inspection equipment, Amusement equipment

Ultra-compact & lightweight

Small packages centered on VSON and S-VSON contributes to the miniaturization and high density of the entire device.

DIP: Assuming the mounting area as 100%, SOP: Mounting area 62%, SSOP: Mounting area 24%, USOP: Mounting area 20%, P-SON: Mounting area 16%, VSON (R): Mounting area 10%, VSON: Mounting area 9%, S-VSON: Mounting area 8% * 84% for VSON

Long life

The contactless structure with optical signal transmission system eliminates life deterioration due to contact wear and tear, contributing to reduced maintenance frequency.

Stable ON resistance

The contactless structure with optical signal transmission system eliminates mechanical wear and tear phenomena, thus maintaining stable resistance values.

Low power consumption

Extremely low power consumption on the input side contributes to energy saving of equipment.

Quiet operation

The contactless structure eliminates opening and closing noises when in use, contributing to quiet operation of equipment.

Fast response

Fast response with an operating time of 0.2 ms (SSOP, USOP, VSON).

Load voltage

Compact yet capable of switching high-voltage loads, contributing to space saving even in high-voltage circuits.
The lineup includes products with a maximum load voltage of 600V.

Dielectric strength

The input-output withstand voltage of 2500 VAC is secured, and a series of 5000V products with high withstand voltage is also available.

For further details, please refer to the following.

Common Terminology in MOS FET Relay Product Catalog

Explains the terms used in the MOS FET relay datasheet.

Click here for the relay product lineup

Product Lineup of Relays