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Solid State Relay Vs MOSFET – What are their Similarities and Differences?

Most times people interchange a MOSFET for a solid state relay (SSR). However, these two components differ based on the way they operate. In this article, we will explore what each component offer, their similarities and differences.

What is the Function of a Solid State Relay?

Solid state relay functions as an electrical switch that can turn off or on an electrical appliance using another electrical input. It makes use of optoelectronics and semiconductor switches rather than using moving contacts and magnetism. We have two different types of solid state relay. The first one switches AC loads and the other type switches DC loads. When there is an AC supply, a DC SSR will go off for half the time. This could result in half-wave rectified current.

When AC relay has a DC supply and there is flow of current, this won’t stop even after removing the control signal. Solid state relay (SSR) is less vulnerable to environmental factors like external magnetic fields, mechanical shock, and vibration. Therefore, SSRs can be relied on. They offer increased lifespan which makes them very reliable.

While the average lifespan of an EMR is one million cycles, an SSR’s lifespan is almost 100 times of that. We can all see that an SSR can stand the test of time. Also, a solid state relay has low emissions, no electrical arcing, and no contact bounce. An SSR with a full rated current produces almost 10X more heat an electromechanical offers.

Therefore, there is a need for good thermal management when using SSR while electromechanical relays don’t need that.

What are the Uses of Solid State Relay?

What is a Relay

A solid state relay serves many purposes in different applications. For instance, this relay is commonly used in industrial applications in which issues about high inrush currents and high switching frequencies arise. Also, SSR can control valves, motors, and electromagnets used in automation, machine, rail, and energy applications.

Furthermore, applications which require quick, frequent switching needs  a solid state relay. For instance, an industrial oven has a temperature control in which the main supply to the heating element will need to be switched off in every two to three seconds. Therefore, an SSR can serve as a switch in this application.

Another important use case of solid state relays is in interfacing. Small SSRs consume a few mA and as such, they absorb the limited output currents from PLCs. You will find SSRs in almost all applications these days. They are available in office machines, computers, industrial controls, and consumer electronics.

Solid state relays have a lot of advantages. These relays function at extremely high speeds and don’t generate any sparks. Due to this, SSRs can function well in explosive atmospheres.

What are MOSFETs?

MOSFET means metal-oxide-semiconductor field-effect transistor. This type of transistor is functional in many electronic devices like computers, televisions, and mobile phones. Silicon, which is a semiconductor material and a thin layer of metal oxide is used in making MOSFETs. The metal oxide which is on the surface of MOSFETs functions as an insulator.

This semiconductor material creates a drain region. MOSFETs comprise a metal gate electrode in which a thin oxide layer insulates it from the semiconductor. Also, their source and drain terminals constitute doped semiconductor material, while metal is used for making the gate terminal. An electric field forms through the application of voltage to the gate terminal. The electric field regulates current flow between the drain and source terminals.

MOSFETs  are devices controlled by voltage. This shows that the voltage used by the gate terminal determines the current flow between the drain and source terminals. As a result of this, MOSFETs are functional devices in electronic applications like digital logic circuits, switching, and amplification. Also, they feature high input-output isolation, low power consumption, and high input impedance.

MOSFETs are more ideal for use in applications where low level of noise, fast switching speed, and high frequency operation are required. MOSFET are high power electrical switches that don’t need physical contact to function. MOSFET can switch and connect signals. They have semiconductor features and mechanical relays. MOSFETs are ideal for use in security equipment, semiconductor inspection system, and other applications.

They are now commonly used for signal switching and amplification. Also, these devices are common in analog and digital circuits. As a three terminal device, MOSFET comprises a Drain (D), a Gate (G), and a Source (S).

What are the Types of MOSFETs?

There are two main types of MOSFET. These are depletion MOSFET and enhancement MOSFET.

Depletion MOSFET

Theree is a channel developed in the Depletion MOSFET during its fabrication process. This means there is a channel even when there is no voltage. As a result of this, the depletion MOSFET is capable of conducting current between drain and source.

The depletion MOSFET can either be  be ‘P-channel or ‘N-channel D-MOSFET’ based on the channel you are using. The type of channel in the depletion MOSFET impacts its biasing, current capacities, and speed.

Enhancement MOSFET

No channel is developed in the enhancement MOSFET during its fabrication. A channel is rather developed in the substrate by the application of voltage via its electrode. The conducting ability of this MOSFET is enhanced by the voltage. This is the reason it is called ‘Enhancement MOSFET.’

Similarities between Solid State Relay Vs MOSFETs


MOSFETs which stands for Metal-Oxide-Semiconductor Field-Effect Transistor and solid state relays share some similarities. These two electronic devices are both capable of regulating electricity flow in a circuit.

Below are some of the similarities these two devices share:

  • MOSFETs and solid state relay can switch electrical circuits on and off. They can as well regulate where a current is flowing to.
  • Both  devices can offer electrical isolation between various parts of a circuit.
  • Electrical signals like current or voltage can control MOSFETs and solid state relay.
  • Both devices function well in several applications like consumer electronic,  industrial control systems, telecommunication systems, and automotive systems.

Solid State relays and MOSFETs are capable of handling high power in devices and as well function in high voltages. However, relays perform better in high current applications when compared to MOSFETs.

Difference Between Solid State Relay Vs MOSFET

Although MOSFETs and solid state relays share some similarities, they differ in some ways. MOSFETs serve different functions, however, they are mainly designed to function as an electrical switch. While a solid state relay is a passive component, a MOSFET is an active component.

The switching speed of MOSFET is much higher than that of a solid state relay. In terms of output current, a MOSFET differs from an SSR. A solid state relay is a more affordable option than MOSFETs. MOSFETs  are devices controlled by voltage.

Another difference between a solid state relay vs MOSFET can be seen in their size. When compared to SSR, a MOSFET is smaller in size. Therefore, MOSFET is ideal for use in applications that require a compact enclosure or circuit.

What are the Advantages of Solid State Relay?

Fast Switching Speed

The switching speed of solid state relay is very fast. Solid state relay don’t have mechanical parts for movement but they have the ability to switch ver fast.

Due to solid state relay’s fast switching speed, it is useful in a wide range of applications, particularly where high frequencies and critical timing are of great importance. For instance, you will find SSRs in the automation systems of homes, communication equipment, and robotics.

No voltage switching

SSRs can be turned on and off at no voltage. This indicates that they don’t have arcing across contacts, unlike electromechanical relay. Putting and contact points’ erosion can occur due to electrical arcing. This may lead to failure in the long run. Since a solid state relay has no arcing problem, it usually lasts longer. Also, SSR operates in flammable environments. There will be a reduction in EMI due to lack of arcing.

Long Lifespan

Since there are no moving parts in solid state relays, they won’t wear out or break. Therefore, this makes them last longer compared to electromechanical relays. The average lifespan of an SSR is millions of cycles and about hundreds of millions hours when used in normal conditions. This accounts for more than 20 years of operation.

Electrical Isolation

The majority of solid state relays produced today isolate control signal from switching circuit. Electrical isolation offers many benefits, of which the most evident ones are arc avoidance and control circuit safety.

The occurrence of electrical sparks or shocks or sparks in SSR,  even when the load generates a high reverse current.

Disadvantages of Solid State Relays


Limited Current and voltage ratings

The current and voltage ratings of solid state relays is a big disadvantage. Most solid state relays can switch low-current and low-voltage loads. There are SSRs that can handle higher current and voltage, however, they can be more expensive compared to the standard types.


Solid State relays are expensive. The use of semiconductor materials contributed to their high cost. Also, their manufacturing processes tend to be complex and as such, this contributes to their high cost.

Output Leakage Current

Small amount of leakage current moves through the output when you turn off a solid state relay. Although, this is a normal occurrence, it can be a problem when the load is vulnerable to small changes in current.


SSRs and MOSFETs have various functions. These devices have proved to be very useful in different applications. They have some similarities and differences. The type of application you are using these devices for will determine which is perfect.




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