There are many different variations of the transistor, and two of them which are used for high power applications are the MOSFET and IGBT.

Like every other electrical and electronic component, transistors have key characteristics that determine how they function. 

While both of them share many similarities, they do differ in many of these key characteristics which include;

• Switching frequency
• Voltage limits
• Current limits 
• Construction
• Application 
• Input impedance
• Gate control signal

These are just some of the many differences. 

This article will take a closer look at MOSFETs, IGBTs and the differences between the two.

What are transistors

Since a MOSFET and IGBT are part of the transistor family, it will help to first learn a little more about the transistor. 

However, if you are familiar with transistors, MOSFETs and IGBTs you can skip these sections straight to the differences part of the article. 

As we just saw above, transistors are a type of semiconducting device, which in essence have the function of the humble switch.

They can be used to either conduct  or insulate current or voltage.

Another neat function of a transistor is the amplification of current or voltage. 

So it can function either as a switch or amplifier. 

Construction of a transistor  

Construction of a transistor commonly consists of placing an oppositely doped semiconductor between two similarly doped semiconductors. 

They commonly consist of three terminals.

One pair of this set of three is where the control signal is applied. The control signal can either be a voltage or current.

The other pair is where the output current flows through. 

Types of transistor

Transistors have two major variations;

• BJT
• FET

BJT

BJT stands for Bipolar Junction Transistor. 

Its three terminals are the Base, Collector, and Emitter.

This is a current controlled transistor, which means a small current at the input allows a larger current to flow at the output.

A BJT can be further divided into two classes; PNP and NPN.

FET  

FET stands for Field Effect Transistor.

The three terminals of a FET include the Gate, Drain and Source.

This type of transistor is voltage controlled, which means a small voltage at the input allows for a larger current to flow at the output. 

A FET has many variations which include;

• MOSFET
• IGBT
• JFET
• ITFFET
• FREDET
• OFET

What is a MOSFET

The first most common variation of the FET is the MOSFET which stands for Metal Oxide Semiconductor Field Effect Transistor. 

MOSFETs are three terminal devices consisting of a Gate (G), Source (S) and Drain (D).

There are two main two groups of MOSFET (Enhancement and Depletion),which can either be N-Channel or P-Channel. This gives us four variations of the MOSFET which include;

• Enhancement Mode N-Channel
• Enhancement Mode P-Channel
• Depletion Mode N-Channel
• Depletion Mode P-Channel 

Construction of a MOSFET

In Enhancement type MOSFETs, there is no physical connection between the source and drain, hence the broken lines in its symbol. 

On the other hand, depletion MOSFETs have a small semiconducting strip that connects the source and drain terminals. 

Below is an image that shows the basic construction of the MOSFET.

The Gate terminal is connected to a substrate where an oxide layer (SiO2) is placed. The purpose of the oxide layer is to act as an insulator. 

The general construction of a MOSFET consists of a lightly doped substrate which is diffused by a heavily doped region. 

The substrate used ultimately determines whether it is a P-type or N-type MOSFET. 

As we saw above, MOSFETs are either Enhancement or Depletion.

N-Channel Enhancement MOSFET

The source and drain in a N-Channel Enhancement MOSFET consist of a N-type semiconductor which is heavily doped and the substrate is a P-type semiconductor. 

Electrons are the main charge carriers. 

P-Channel Enhancement MOSFET

On the other hand, the source and drain of P-Channel Enhancement MOSFETs are of a P-type semiconductor which is heavily doped, and the substrate is a N-type semiconductor. Holes are the major carriers here.

In enhancement mode, the source and drain are isolated. 

N-Channel Depletion MOSFET

In N-channel depletion MOSFETs the source and drain are connected by a small strip of material that is a semiconductor of N-type. The substrate is a P-type semiconductor. 

The main charges are electrons while the source and drain are heavily doped.

P-Channel Depletion MOSFET

The P-channel depletion MOSFET is the opposite of the N-channel, as the strip of semiconducting material connecting the source and drain is P-type. The substrate is N-type. 

The main carriers are holes.

What is an IGBT

The next most common variation of a FET is the IGBT which stands for Insulated Gate Bipolar Transistor.

However, while it falls under the umbrella of a FET, the IGBT is a mashup of a MOSFET and BJT.

It combines the best parts of both these transistors to achieve high input impedance and switching speeds (of a MOSFET), as well as low voltage saturation (of a BJT). 

IGBTs are aptly named because of this mashup of technologies. 

They get the insulated gate technology from a MOSFET, and the output performance characteristics from a BJT.

So, you end up getting a voltage controlled transistor, with the output characteristics of a BJT. 

It too is a three terminal device which consists of a Gate (G), Emitter (E) and Collector (C). 

Construction of  IGBT

We now know that the IGBT is a combination of a MOSFET and BJT which you can see physically in the way it is constructed. 

It combines an N-channel MOSFET at the input, with PNP type BJT at the output. They are connected in a Darlington configuration.

This is why the input terminal is called the Gate, and the output terminals Collector and Emitter. 

Below is an image of the internal structure of an IGBT. 

An IGBT consists of four layers of semiconducting material. This gives it a PNPN structure. 

The collector terminal is fixed to the P-Layer, and the emitter is connected between the P and N layers. 

MOSFET vs IGBT

So now that we have taken a closer look at a MOSFET and IGBT we can discuss the major differences between the two transistors.

While they fall under the same umbrella of transistors (FET), they do vary in many characteristics as we shall soon see.

Knowing these differences will help you decide if one is more suited for a certain application compared to another. 

Let’s take a look.

MOSFET vs IGBT difference #1: Construction

Right off the bat we can see that the first major difference between the two transistors is their physical construction.

Both devices are three terminal devices, however, the IGBT combines the structures of a MOSFET and BJT which give it a set of unique qualities.

MOSFET vs IGBT difference #2: Voltage

MOSFETs and IGBTs are used in applications where they are used to isolate devices from logic circuits. 

In these circuits the device in question is operating at a voltage that is higher than that of the voltage that the logic circuit is operating at. 

Devices connected at the output of a MOSFET and IGBT can range in voltages from low to very high. 

The amount of voltage that these devices operate at will determine how much voltage the MOSFET or IGBT will be subjected to.

So the right component will be needed for the job.

When it comes to voltages, IGBTs are best suited for high voltage applications as they can handle voltages in excess of 1000 volts.

For low voltage applications, a MOSFET is a more suitable option. They are best suited for voltages less than 250 volts. 

MOSFET vs IGBT difference #3: Current

The next major difference between the MOSFET and IGBT is their current capabilities.

Another key factor when looking at devices connected to the output of transistors, is how much current they require to operate efficiently. 

Just like voltage, you will need to know the maximum current that a MOSFET and IGBT will be subjected to. 

As operating outside their maximum range will damage them. 

Again, IGBTs are best suited for high current applications, whereas MOSFETs are a better choice for applications with low currents. 

MOSFET vs IGBT difference #4: Switching frequency

If you were to close and open a mechanical switch, there is going to be a delay. You will not be able to switch between the two states instantaneously.

This is true for the switching of MOSFETs and IGBTs as well.

The time of how long it takes to switch-on and switch-off  MOSFETs and IGBTs is known as switching frequency. 

Switch-on times tell us the delay before current starts flowing, while switch-off times tell us the delay when current stops flowing. 

MOSFETs dominate in the switching frequency department with the capability to exceed values of greater than 200kHz.

IGBTs however, have much lower switching frequency values that range less than 20kHz.

MOSFET vs IGBT difference #5: Unidirectional vs Bidirectional

When enough voltage is applied to the gate terminal of both devices, current starts to flow through their outputs (from source to drain in a MOSFET, and collector to emitter in an IGBT).

If current is only able to flow through a transistor in one direction (forward), it is considered to be a Unidirectional device.

However, if current can flow in both directions (forward and reverse), the transistor is now known as a Bidirectional device. 

IGBTs only have the capability to switch current in one direction (forward), making them a unidirectional transistor. 

MOSFETs can switch current in both directions (forward and reverse), making them a bidirectional transistor.

But, while the switching of current in MOSFET in the forward direction is controlled, it is uncontrolled in the reverse direction. 

MOSFET vs IGBT difference #6: Power gain

Transistors are versatile devices and their unique characteristics can be utilised in many applications. 

While they are primarily used as switches, transistors can also be used as an amplifier. 

An amplifier has the ability to ‘amplify’ a weaker signal into a much more powerful one. 

MOSFETs and IGBTs have a similar ability. A change of voltage at the input (gate) creates a huge current at their respective outputs. 

So, both can amplify a weak signal into a more powerful one just like an amplifier. 

One important and vital characteristic of an amplifier is its Power Gain which tells us the ratio of power at the output over the power at the input. 

The higher the power gain the better, as weaker signals can be made into stronger ones. 

When it comes to power gain for the MOSFET and IGBT, the IGBT wins this battle having a much higher power gain. 

MOSFET vs IGBT difference #7: Conduction and Switching losses

In an ideal world, power loss would not be an issue. 

However, we do not live in an ideal world so we have to deal with power loss. When it comes to MOSFETs and IGBTs, there are two major types of power losses ; conduction loss and switching loss

Conduction loss in a transistor occurs when it is conducting current, and is a product of current and voltage. 

Switching loss occurs when the transistor is switching between ON and OFF states and is dependent on the duty cycle.

A MOSFET has higher conduction losses compared to the IGBT, while the IGBT has higher switching losses compared to the MOSFET. 

MOSFET vs IGBT difference #8: Bipolar vs Unipolar

Earlier we saw transistors can be either unidirectional or bidirectional. The final major difference between a MOSFET and IGBT is whether it is Unipolar or Bipolar.

A bipolar transistor is a type of transistor that uses both electrons and electron holes as charge carriers, while a unipolar transistor only uses one type of charge carrier (either electron holes or electrons).

Due to its construction, an IGBT is a bipolar device using both electrons and electron holes as charge carriers. 

MOSFETs are unipolar devices as they only use one type of charge carrier. 

Summary of differences between a MOSFET and IGBT

If you don’t have the time  to read the details of the aforementioned differences between the MOSFET and IGBT, below is a table with the summary (plus some additions).

Similarities between a MOSFET and IGBT

So we have just seen the major differences between the MOSFET and IGBT. But, do these transistors share any similarities?

Yes!

The most common similarity that these two transistors share is the fact that both of them are Voltage Controlled.

For a transistor to start conducting current at its output it requires a control signal at its input. This control signal can either be a current or a voltage. 

If current is the parameter that controls the transistor, it is considered to be a current controlled device. But, if voltage is the controlling parameter, it is a voltage controlled device. 

Since the MOSFET and IGBT both use a voltage at the input they are voltage controlled devices.  

Are MOSFETs and IGBTs interchangeable?  

While both transistors are used for similar applications, they do have differences as we just saw which ultimately will be a deciding factor if you can replace one with another.

Let’s take a look at some of these factors. 

First is the voltage and current capability of these devices. 

MOSFETs are on the lower range of voltage and current capabilities, while IGBTs have higher limits. 

So, you would not be able to replace an IGBT with a MOSFET as the MOSFET won’t have the ability to handle these high voltages and currents. 

On the other hand, an IGBT would be able to replace a MOSFET, but doing so is overkill. This would be analogous to cutting a slice of bread with a saw. 

While possible, it probably isn’t the best idea.

A better option would be to replace multiple MOSFETs with a single IGBT.

The next factor is a parasitic body diode. This is only common with MOSFETs due to its structure. The parasitic body diode is formed between the source and drain.

This parasitic diode is made use of in many applications, especially when it comes to motor drive circuits. 

If you replace a MOSFET with an IGBT in these types of circuits, you will be losing this essential parasitic body diode.

You will need to add an additional parallel diode to combat this issue. 

Other factors to consider include;

• IGBT have high gate capacitance and require large current to turn on and off
• When it comes to switching applications, MOSFET can switch faster with lower loss. If you replace it with an IGBT but keep switching speeds high, you will have higher power losses. 

Which is better, a MOSFET or IGBT?

So the MOSFET and IGBT aren’t entirely interchangeable. 

There are many factors to consider.

But, if you had to choose, which is the better option?

Deciding on which is better really comes down to the needs of the application. It cannot be said that one  is better than the other.

You have to assess the application to see what its needs are and then choose the right transistor for the job. 

One transistor will be more suitable for a particular application compared to the other. But this doesn’t mean it is better overall.

For example, an IGBT is more suitable for high voltage applications compared to a MOSFET.

The post MOSFET vs IGBT - 8 Key Differences appeared first on Electronic Guidebook.

Every option however, will have its own advantages and disadvantages. 

The two options at the centre of this article are a MOSFET and a Relay.

So, MOSFET vs RELAY, which is the better option?

There are many considerations to examine when pondering the above question. While both have a similar purpose, they do have their differences. 

This article shall take a closer look at the MOSFET and Relay, and their differences to see which might be best suited for a particular application compared to another.

Deeper look at a MOSFET and Relay

Before we can put the MOSFET and Relay head to head, it will benefit us if we first learn a bit more about each component (it will be brief).

If you are already well versed about each component, you can skip this section. 

What is a Relay?

Since the relay came first, we can start our journey with it. 

There are many applications where a logic circuit of low current controls an output circuit which is subject to higher currents. 

In these scenarios it is important to isolate the control circuit from the output as the large currents could damage the logic circuit. 

This is where the Relay comes to the rescue. 

A relay is a device that operates like a connection between two circuits of different current levels. Relays control one electrical circuit by opening and closing its contacts in another circuit 

They essentially convert a small input current into a high output current. 

Earlier relays were electromechanical which meant that there was physical contact when they were opened and closed. 

However, as with most things, the relay evolved and now comes in a Solid-State version which does the switching with no physical contact (everything is achieved electronically). 

But one is not better than the other as each has its own purpose for the needs of certain applications (a discussion for another day). 

When comparing the MOSFET and Relay, we shall concentrate on the earlier electromechanical version of the relay. 

Relay construction and working principle

If we were to open up a relay and break it down, we would be able to separate it into four major components which include;

1. Frame 
2. Coil 
3. Armature
4. Contacts 

Frame – this is part of the relay that encompasses and supports the other parts

Coil – has the main function of pulling the armature by means of a magnetic field (the armature returns to its original position by means of a spring after the magnetic field is removed).

Armature – the main moving part of the relay which has contacts to the ends.

Contacts – part of the relay that provides a physical path for current to flow. 

Relays consist of two circuits, an energising circuit which is connected to the control circuit, and a contact circuit which is where the output gets connected. 

When the coil of the relay is energized, a magnetic field is generated thus pulling the armature which closes the circuit of the output allowing current to flow. 

What is a MOSFET?

Transistors are semiconducting devices that can be used either to amplify or switch electrical signals. 

The MOSFET is part of the family of transistors which shares the same purpose as its peers. But, the way it goes about doing so is different. 

Furthermore, it is a subset of another commonly used transistor, the Field Effect Transistor (FET).

MOSFET stands for Metal Oxide Semiconducting Field Effect Transistor. 

It is a three terminal device which includes a Gate(G), Source (S), and Drain(D).

There are two main two groups of MOSFET (Enhancement and Depletion),which can either be N-Channel or P-Channel. This gives us four types of MOSFET seen below;

• Enhancement Mode N-Channel
• Enhancement Mode P-Channel
• Depletion Mode N-Channel
• Depletion Mode P-Channel 

The main purpose of a MOSFET

MOSFETs have a wide variety of uses , but ultimately they were created with one specific purpose, to act as an electrical switch. 

A mechanical switch is a device used for making or breaking a connection within an electric or electronic circuit. 

When the switch is ‘OPEN’, no current can flow through the circuit. The circuit is said to be ‘OFF’.

When the switch is ‘CLOSED’, current can now flow through the circuit powering whatever load is connected. The circuit is said to be ‘ON’.  

A MOSFET functions in a similar fashion, but does so electrically. 

When a MOSFET is in the cut-off region, no current can pass through it (just like a switch when it is OPEN). 

But, when it is in the saturation region, current can freely flow (just like when a switch is CLOSED). 

So, in the cut-off region it is effectively OPEN inhibiting the flow of current, and in the saturation region it is CLOSED allowing the current to flow just like a switch. 

However, the MOSFET has greater capability than a normal switch. It also has the ability to isolate high currents in its output from low current circuits at its input. 

What do MOSFETs and Relays have in common?

While a MOSFET and Relay have many differences, they do have one thing in common which is to provide isolation between low current and high current circuits. 

They act as a ‘switch’ allowing for a small current at the input to control a larger current at the output. 

Differences between a MOSFET and Relay

So, the MOSFET and Relay share a common goal. However, while their goals might align, the way they go about achieving them varies.

This section shall discuss the key differences between MOSFETs and Relays. 

Let’s take a look.

Difference #1 between a MOSFET and Relay: Output Current Limit

The first major difference between a MOSFET and Relay is the output current limit. 

Both devices have many important characteristics and none more so than the amount of current that they can handle at their output. 

There are many devices that can be connected at the output of MOSFETs and Relays. Devices such as motors, lights, speakers etc.

Depending on the complexity of the device, as well as its size, it will require a certain amount of current to operate. 

Bigger devices will require higher current levels to operate. MOSFETs and Relays will need to be able to handle these high currents without failure.

When it comes to output current limits, Relays have a higher current limit compared to MOSFETs making them more suited for high current applications. 

Difference #2 between a MOSFET and Relay: Isolation

The main purpose of both devices is to provide isolation between the input and output circuits. The input circuits deal with lower current levels than the output.

As we just saw, the magnitude of current at the output can be quite large. If this current has a path to the input, all components on the input side will be damaged. 

So, it is important for both the MOSFET and Relay to have Isolation between the input and output circuits. 

Relays provide complete electrical isolation compared to MOSFETs further making them a better option for high current applications as these currents have no path to the input.

Difference #3 between a MOSFET and Relay: Cost

Money is a big factor when it comes to engineering projects. You might be given the task of designing and creating a certain circuit and are only allotted a fixed amount of money.

It will be beneficial to you to be able to choose the cheapest components (without sacrificing quality of course), so that you have enough money to buy all necessary parts. 

If the application you are designing for requires the ability of the MOSFET or Relay, but you are strapped for cash, the relay is a better option as it tends to be cheaper. 

Difference #4 between a MOSFET and Relay: Active vs Passive

Electrical and electronic components can be split up into two categories; Active and Passive.

Active components have the ability to produce power in the form of voltage or current, while Passive components do not have the ability to produce power but rather store power in the form of voltage or current.

The next difference is that a MOSFET is an active device, while a Relay is passive. 

Difference #5 between a MOSFET and Relay: Linear vs Non-Linear

Just like whether a component is active or passive, they can be further divided up into whether they are Linear or Non-Linear.

Linear components have a steady change in current as a changing voltage is applied to them. The flow of current changes linearly. 

On the other hand,  the flow of current through Non-linear components does not happen linearly with a changing voltage across them (hence why they are called non-linear).

In this instance, MOSFETs are non-linear devices, while Relays are linear. 

Difference #6 between a MOSFET and Relay: Switching times

If you were to close and open a mechanical switch, there is going to be a delay. You will not be able to switch between the two states instantaneously.

This is true for the switching of MOSFETs and relays as well.

The times of how long it takes to switch-on and switch-off  MOSFETs and relays is known as switching times. 

Switch-on times tell us the delay before current starts flowing, while switch-off times tell us the delay when current stops flowing. 

Due to no moving parts in MOSFETs, their switching times are faster compared to relays. 

Also, due to their mechanical nature, relays have issues with contact bounce associated with the mechanical contacts. 

Difference #7 between a MOSFET and Relay: Lifetime

How long something lasts is of great importance no matter what area you might be discussing. It would suck if your car only lasted a year before you had to buy a new one.

The same holds true for components in the electrical world. 

The longer they can last the better!

Now when it comes to the lifetime of a MOSFET vs a Relay, due to the fact that the MOSFET has no moving parts, it has an increased lifetime as opposed to a relay.

As you can imagine, the physical movement within the relay is going to generate a lot of wear and tear greatly reducing its lifetime. 

Difference #8 between a MOSFET and Relay: Size

Another important parameter that needs to be taken into consideration when designing circuits is Size. 

The application might demand the need for a compact circuit and enclosure. If the physical dimension of the printed circuit board decreases, you are going to have less space to play with. 

If you are able to find components of smaller physical size this will benefit you.

MOSFETs are notably smaller in size compared to relays.  

MOSFET vs Relay ; Which is the better option?

So we have just covered the notable differences between a MOSFET and Relay. 

But, when we put them head to head, is one better than the other? 

The simple answer is no, one device is not better than the other. This is because there are a myriad of electrical and electronic applications each with their own unique needs. 

And a MOSFET or a relay will not be able to meet the needs of every particular application.

For example, if an application will be subject to large amounts of current, and switching needs to be done at relatively low speeds, a relay is the perfect candidate.

However, if now the application requires high switching speeds, and current is moderate, a MOSFET is the perfect option.

So you see, it really depends on the application’s needs.

Also, MOSFETs and relays are not always interchangeable.

The post MOSFET vs Relay - 8 Key differences appeared first on Electronic Guidebook.

Like many electrical and electronic components, MOSFETs have many important characteristics that determine how it operates.

One of these characteristics is the Threshold Voltage (Vth).

So what exactly is the threshold voltage of a MOSFET? The threshold voltage of a MOSFET is the minimum gate-to-source voltage (Vgs) required to turn the MOSFET ‘ON’ (which allows current to start flowing between source and drain).

However, there are variations of  MOSFET each having different threshold voltages.

This article shall take a closer look at the MOSFET, its variations and threshold voltage. 

What is a MOSFET?

Transistors are semiconducting devices that can be used either to amplify or switch electrical signals. 

The MOSFET is part of the family of transistors which shares the same purpose as its peers. But, the way it goes about doing so is different. 

Furthermore, it is a subset of another commonly used transistor, the Field Effect Transistor (FET).

MOSFET stands for Metal Oxide Semiconducting Field Effect Transistor. 

It is a three terminal device which includes a Gate(G), Source (S), and Drain(D).

There are two main two groups of MOSFET (Enhancement and Depletion),which can either be N-Channel or P-Channel. This gives us four types of MOSFET seen below;

• Enhancement Mode N-Channel
• Enhancement Mode P-Channel
• Depletion Mode N-Channel
• Depletion Mode P-Channel 

Construction of a MOSFET

In Enhancement type MOSFETs, there is no physical connection between the source and drain, hence the broken lines in its symbol. 

On the other hand, depletion MOSFETs have a small semiconducting strip that connects the source and drain terminals. 

Below is an image that shows the basic construction of the MOSFET.

The Gate terminal is connected to a substrate where an oxide layer (SiO2) is placed. The purpose of the oxide layer is to act as an insulator. 

The general construction of a MOSFET consists of a lightly doped substrate which is diffused by a heavily doped region. 

The substrate used ultimately determines whether it is a P-type or N-type MOSFET. 

As we saw above, MOSFETs are either Enhancement or Depletion.

N-Channel Enhancement MOSFET

The source and drain in a N-Channel Enhancement MOSFET consist of a N-type semiconductor which is heavily doped and the substrate is a P-type semiconductor. 

Electrons are the main charge carriers. 

P-Channel Enhancement MOSFET

On the other hand, the source and drain of P-Channel Enhancement MOSFETs are of a P-type semiconductor which is heavily doped, and the substrate is a N-type semiconductor. Holes are the major carriers here.

In enhancement mode, the source and drain are isolated. 

N-Channel Depletion MOSFET

In N-channel depletion MOSFETs the source and drain are connected by a small strip of material that is a semiconductor of N-type. The substrate is a P-type semiconductor. 

The main charges are electrons while the source and drain are heavily doped.

P-Channel Depletion MOSFET

The P-channel depletion MOSFET is the opposite of the N-channel, as the strip of semiconducting material connecting the source and drain is P-type. The substrate is N-type. 

The main carriers are holes.

Regions of operation of a MOSFET

The MOSFET has  three major areas of operation which include;

• Cut-Off region
• Saturation Region
• Linear/Ohmic Region

Cut-off region

When the MOSFET is in the cut-off region the device is considered to be in its ‘OFF’ state. When it is OFF, no current flows through it. 

Just like when a mechanical switch is open and stops the flow of current as well.

Saturation region

Saturation is when the current flow through the MOSFET is a constant value. In this region it behaves like a closed switch allowing current to flow freely. 

Linear/Ohmic region

Last but not least is the Linear/Ohmic region. 

In this region of operation, an increase of voltage across the drain and source sees an increase in current through the MOSFET as well.

The threshold voltage of the MOSFET

When there is no voltage present between the gate and source terminals, the MOSFET is operating in the cut-off region. 

In this region the MOSFET is ‘OFF’ as no current can flow from the source to drain.

In order to allow current to flow freely from the source to drain, we need a conducting path. This conducting path is created when the MOSFET is operating in the saturation region.

But in order to get to the saturation region we need to turn the MOSFET ‘ON’.

To do so we need to apply a sufficient amount of voltage between the gate and source terminals known as VGS. 

The minimum amount of voltage required to enter the saturation region and start conducting current is known as the Threshold Voltage or VTH. 

When VGS > VTH, the MOSFET is now said to be ‘ON’.

Threshold voltage for the different types of MOSFET

We know that there are derivatives of the MOSFET. 

But, are the threshold voltages the same for all four?

The table below summarises the conditions for the different threshold voltages for the various types of MOSFET.

*The information in this table is thanks to https://www.electrical4u.com/mosfet-characteristics/ . For a more in-depth explanation check them out. 

VTH = threshold voltage

VGS = gate-to-source voltage

VDS = drain-to-source voltage 

VP =  pinch-off voltage

What is the threshold voltage required to turn a MOSFET on?  

So, how much voltage does it take to put the MOSFET on? 

This comes down to the two main regions of operation of a MOSFET;

• Cut-Off
• Saturation 

As we just learnt, when the MOSFET is ‘OFF’, it is in the cutoff region.

The saturation region is when the MOSFET is ‘ON’ and current can flow freely from source to drain. This is when the maximum gate voltage is applied and results in the maximum amount of current that can flow. 

To get to this region of operation VGS > VTH.

The exact threshold voltage will differ for the four types of MOSFET, as  there are many factors like materials used, manufacturer, oxide thickness, etc, which will determine the threshold voltage value. 

To find out the exact threshold voltage for a particular MOSFET, you will have to look up its datasheet which will have the necessary values. 

Threshold voltage is usually denoted VGS(TH) in most datasheets. 

Is the threshold voltage of the MOSFET constant?

You might be wondering if the MOSFET is exactly like a switch, in the sense that when it is closed the maximum amount of current can flow.

The answer to that is no. The amount of current that flows is dependent on the amount of voltage applied to the gate and source (Vgs).

So, when there is more to the question of whether threshold voltage is constant or not.

There is a range of threshold voltages that range from minimum to maximum. 

When the amount of Vgs is greater than Vth, this will allow current to start flowing. But, this is not the maximum amount of current that can flow (like when a switch is closed).

The amount of drain current that can flow is determined by how much voltage is applied to the gate and source and also sits between the range of minimum and maximum threshold voltage. 

Lower values of Vgs will conduct less drain current, while higher values will allow higher amounts of current to flow. 

MOSFETs have a Drain Characteristics Curve that shows the amount of current for different Vgs values. 

Factors that can affect the threshold voltage of a MOSFET

Performance of electronic components can be affected by internal or external factors and the MOSFET is no different.

The threshold voltage of MOSFET can be influenced by a few factors which include;

• Oxide Thickness
• Temperature 
• Random dopant Fluctuation 

Oxide thickness

The type of oxide chosen to construct the MOSFET, as well as its thickness plays a large part in the threshold voltage.

Thickness of the oxide layer shares a directly proportional relationship with threshold voltage which means the smaller the oxide layer, the smaller the threshold voltage and vice versa. 

Temperature

While there is no direct relationship the side effects of temperature do affect the threshold voltage with the variation being -4mV/K or -2mV/K which is dependent on the doping level.

A change in temperature around 30 °C can see a change in threshold voltage of around 500mV. 

Random dopant fluctuation

Random Dopant Fluctuation or RDF is a process where the amount of random dopant fluctuation is reduced by lowering the dopant density. 

The RDF in the channel region of MOSFETs can vary certain characteristics, like threshold voltage.

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It is a type of solid-state device used to control the flow of current. 

A MOSFET is one of the many varieties of transistors available that perform the tasks mentioned above. 

But, transistors can either be voltage or current controlled devices.

So, is a MOSFET a voltage or current controlled device? The MOSFET is a voltage controlled device. This is due to the fact that the flow of current between its SOURCE and DRAIN terminals is controlled by an input voltage at its GATE terminal.

This article shall take a closer look at why the MOSFET is a voltage controlled device. 

Difference between a voltage and current controlled device?

To better understand why a MOSFET is a voltage controlled device, it will help to learn what the differences are between a voltage and current controlled device. 

So let’s take a look. 

As mentioned at the start, transistors are a semiconductor device used for the amplification, control, and generation of electrical signals most of  which have three terminals (some have four). 

They are active devices that have many applications. One of the most common being the main component in Integrated Circuits (such as microprocessors and microcontrollers). 

Below are the many different types of transistor;

• BJT 
• FET
• JFET
• IGBT
• MOSFET

While each of these types of transistors share similar functions, the way they go about doing it is a bit different. 

Some of them are voltage controlled and the others are current controlled.

What is a voltage controlled device?

A transistor that is voltage controlled, has voltage as the controlling parameter.

What this means is that the flow of current at the output terminals is controlled by a voltage at the input terminal. 

The output current is a function of an input voltage. 

What is a current controlled device?

On the other hand, a transistor that is current controlled, has current as the controlling parameter.

This means that the flow of current at the output terminals is controlled by a current at the input terminal.

Output current is a function of an input current. 

Why a MOSFET is a voltage controlled device

So why is a MOSFET a voltage controlled device?

This comes down to its construction and working principle. Let’s take a closer look at the MOSFET and what makes it a voltage controlled device.

What is a MOSFET?

We learnt earlier that transistors are semiconducting devices that can be used either to amplify or switch electrical signals. 

The MOSFET is part of the family of transistors which shares the same purpose as its peers. But, the way it goes about doing so is different. 

Furthermore, it is a subset of another commonly used transistor, the Field Effect Transistor (FET).

It is a three terminal device which include a Gate(G), Source (S), and Drain(D).

There are two main two groups of MOSFET (Enhancement and Depletion),which can either be N-Channel or P-Channel. This gives us four types of MOSFET seen below;

• Enhancement Mode N-Channel
• Enhancement Mode P-Channel
• Depletion Mode N-Channel
• Depletion Mode P-Channel 

Construction of a MOSFET

In enhancement type MOSFETs, there is no physical connection between the source and drain, hence the broken lines in its symbol. 

On the other hand, depletion MOSFETs have a small semiconducting strip between the source and drain terminals. 

Below is an image that shows the basic construction of the MOSFET.

The Gate terminal is connected to a substrate where an oxide layer (SiO2) is placed. The purpose of the oxide layer is to act as an insulator. 

The general construction of a MOSFET consists of a lightly doped substrate which is diffused by a heavily doped region and the substrate used ultimately determines whether it is a P-type or N-type MOSFET. 

As we saw above, MOSFETs are either Enhancement or Depletion.

N-Channel Enhancement MOSFET

The source and drain in a N-Channel Enhancement MOSFET consist of a N-type semiconductor which is heavily doped and the substrate is a P-type semiconductor. Electrons are the main charge carriers. 

P-Channel Enhancement MOSFET

On the other hand, the source and drain of P-Channel Enhancement MOSFETs are of a P-type semiconductor which is heavily doped, and the substrate is a N-type semiconductor. Holes are the major carriers here.

In enhancement mode, the source and drain are isolated. 

N-Channel Depletion MOSFET

In N-channel depletion MOSFETs the source and drain are connected by a small strip of material that is a semiconductor of N-type. The substrate is a P-type semiconductor. 

The main charges are electrons while the source and drain are heavily doped.

P-Channel Depletion MOSFET

The P-channel depletion MOSFET is the opposite of the N-channel, as the strip of semiconducting material connecting the source and drain is P-type. The substrate is N-type. 

The main carriers are holes. 

Working principle of a MOSFET 

While there are different types of MOSFET, their main function remains the same which is to act as a switch.

The main operation goes a little something like this; when a voltage (VGS) is applied at the Gate (which exceeds VTH also known as the threshold voltage) of the MOSFET, the path between Source and Drain closes allowing current to flow through.

VGS is the minimum amount of voltage required to turn the MOSFET “ON” 

There are some slight differences in the operation of Enhancement and Depletion MOSFETs, but their overall functionality remains the same. 

As you can see, to turn the MOSFET ‘ON’ (allow current to flow through the source and drain), a voltage is required at the gate terminals.

In a BJT the input impedance is finite which results in a small finite base current which controls the output current. 

When it comes to the MOSFET, there is no input current due to the SiO2 layer between the gate and body. The output current is controlled by an input voltage.  

This is why a MOSFET is known as a voltage controlled device. 

How much voltage is required to control the MOSFET?

When the voltage at the input of the MOSFET is zero, no current flows at its output (source and drain). When no current is flowing the MOSFET is said to be operating in its Cut-Off region.

So, how much voltage does it take to the MOSFET on? 

This comes down to the two main regions of operation of a MOSFET;

• Cut-Off
• Saturation 

As we just learnt, when the MOSFET is ‘OFF’, it is in the cutoff region.

The saturation region is when the MOSFET is ‘ON’ and current can flow freely from source to drain. This is when the maximum gate voltage is applied and results in the maximum amount of current that can flow. 

To get to this region of operation VGS > VTH.

The threshold voltage (VTH) is never going to be the same for all MOSFETs as they might come from different manufactuers.

However, this value can be found in the datasheet of the MOSFET.

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Another component not mentioned above but very important in many electrical and electronic circuits  is the MOSFET.

But, what can a MOSFET be used as? A MOSFET is primarily used as a switch, but can also be used as a variable resistor, diode, capacitor, relay and amplifier.

This article shall take a closer look at how the MOSFET is used with these different functionalities.

A deeper look at the MOSFET

Before we can discuss the different functionalities a MOSFET can be used as, it will help to first learn a bit more about it.

So let’s take a closer look.

MOSFET is an acronym which stands for; Metal Oxide Semiconductor Field Effect Transistor.

It is a three terminal device consisting of the Source (S), Gate (G), Drain (D).

Types of MOSFET

MOSFETs have many variations which is summarised in the flowchart below.

The symbols for each type of MOSFET is shown in the figure below

Construction of a MOSFET

The MOSFET has a very similar construction to one of its close peers, the FET (field effect transistor). It is made up of a semiconducting material. 

Below is an image that shows the basic construction of the MOSFET.

The Gate terminal is connected to a substrate where an oxide layer is placed. The purpose of the oxide layer is to act as an insulator. 

The general construction of a MOSFET consists of a lightly doped substrate which is diffused by a heavily doped region and the substrate used ultimately determines whether it is a P-type or N-type MOSFET. 

As we saw above, MOSFETs are either Enhancement or Depletion.

N-Channel Enhancement MOSFET

The source and drain in a N-Channel Enhancement MOSFET consist of a N-type semiconductor which is heavily doped and the substrate is a P-type semiconductor. Electrons are the main charge carriers. 

P-Channel Enhancement MOSFET

On the other hand, the source and drain of P-Channel Enhancement MOSFETs are of a P-type semiconductor which is heavily doped, and the substrate is a N-type semiconductor. Holes are the major carriers here.

In enhancement mode, the source and drain are isolated. 

N-Channel Depletion MOSFET

In N-channel depletion MOSFETs the source and drain are connected by a small strip of material that is a semiconductor of N-type. The substrate is a P-type semiconductor. 

The main charges are electrons while the source and drain are heavily doped.

P-Channel Depletion MOSFET

The P-channel depletion MOSFET is the opposite of the N-channel, as the strip of semiconducting material connecting the source and drain is P-type. The substrate is N-type. 

The main carriers are holes. 

Working principle of a MOSFET  

While there are different types of MOSFET, their main function remains the same which is to act as a switch.

The main operation goes a little something like this; when a voltage is applied at the Gate (which exceeds Vgs) of the MOSFET, the path between Source and Drain closes allowing current to flow through.

Vgs is the minimum amount of voltage required to turn the MOSFET “ON” also known as threshold voltage.

There are some slight differences in the operation of Enhancement and Depletion MOSFETs, but their overall functionality remains the same.  

Areas of operation of a MOSFET

Knowing the different regions of operation will greatly help us understand the different functions that the MOSFET can be used as. 

The three major areas of operation of a MOSFET include;

• Cut-Off region
• Saturation Region
• Linear/Ohmic Region

Cut-off region

When the MOSFET is in the cut-off region the device is considered to be in its ‘OFF’ state. When it is OFF, no current flows through it. 

Just like when a mechanical switch is open and stops the flow of current as well.

Saturation region  

Saturation is when the current flow through the MOSFET is a constant value. In this region it behaves like a closed switch allowing current to flow freely. 

This occurs when the voltage across the drain and source exceeds the threshold voltage (Vgs).

Linear/Ohmic region

Last but not least is the Linear/Ohmic region. 

In this region of operation, an increase of voltage across the drain and source sees an increase in current through the MOSFET as well.

What a MOSFET can be used as

Now that we have covered a little bit of the basics of the MOSFET, let’s dive into its different functionalities. 

What a MOSFET can be used as: Switch

The first main function that a MOSFET can be used as is the humble Switch.

What is a switch? 

A switch is a device used for making or breaking a connection within an electric or electronic circuit. 

When the switch is ‘OPEN’, no current can flow through the circuit. The circuit is said to be ‘OFF’.

When the switch is ‘CLOSED’, current can now flow through the circuit powering whatever load is connected. The circuit is said to be ‘ON’.  

How does the MOSFET function as a switch?

Here is how a switch behaves. 

When a MOSFET is in the cut-off region, no current can pass through it (just like a switch when it is OPEN). But, when it is in the saturation region, current can freely flow (just like when a switch is CLOSED). 

So, in the cut-off region it is effectively OPEN inhibiting the flow of current, and in the saturation region it is CLOSED allowing the current to flow just like a switch. 

What a MOSFET can be used as: Variable resistor

Resistors are one the most common and essential components in the electrical and electronic world. 

They are a passive component which add resistance to a circuit thereby limiting the flow of current (not stopping it, but limiting it).

The higher the resistance, the less current can flow, and vice versa.

A variable resistor is a type of resistor which has the ability to vary its resistance (using either a knob, or slider). Current is varied as the resistance of the variable resistor is altered. 

Variable resistors have their own added applications in circuits. 

The next major function of a MOSFET is its ability to be used as a variable resistor. 

We learnt that in the Linear/Ohmic region that the voltage across the source and drain controls the amount of current flow. 

This is essentially what we need if we want the MOSFET to function as a variable resistor as we are able to limit the current using voltage. 

What a MOSFET can be used as:  Diode

The next function that a MOSFET can be used as is a Diode.

A Diode is a semiconducting device which only allows current to flow in one direction. Just like a one way street allows motorists to travel in one direction and not the other.

It provides low resistance in one direction, and has a high resistance in the other. 

To imitate a diode, the Gate terminal of the MOSFET is connected to the Drain.

In this configuration the MOSFET functions as a Diode with similar characteristics to that of a PN-Junction Diode. 

What a MOSFET can be used as:  Capacitor

Another common and important component that you will find in almost all electronic circuits is a Capacitor. .

A capacitor is a two terminal passive device which stores electrical energy in the form of an electrical field. 

They consist of two conducting plates which are close to each other separated by an insulator (air, plastic, mica, etc).

The amount of electrical energy it is able to hold is determined by its capacitance which has the units of Farads (F).

In the ‘OFF’ state, the MOSFET is essentially a capacitor as it has to conducting plates separated by an insulator (in this instance SiO₂ ).  

However its capacitance won’t be as high as the surface area (which plays a big part on the amount of capacitance) of the conducting plates won’t be as high.

Varying the voltage at the gate of the MOSFET below the threshold voltage will change the thickness of the non-conductive gap which helps to create a voltage controlled capacitor. 

What a MOSFET can be used as:  Relay

Some applications require an isolation between the logic/control circuit and the load circuit. This is due to the fact that the load circuit might be subject to higher currents which could damage components and devices in the control circuit.

A relay is an electrically operated switch that enables the separation of two separate circuits.It allows you to link different signal levels and different potentials without any issues. 

The opening and closing of circuits comes in the form of metallic contacts which are opened and closed using an electromagnet contact. 

MOSFETs operate in a very similar fashion to a relay. While relays provide a mechanical means of isolation, MOSFETs provide electrical isolation.

MOSFETs can operate high currents between source and drain without affecting the control circuit which the gate is connected to. 

They are commonly used as solid state relays. 

What a MOSFET can be used as:  Amplifier

Amplifiers are aptly given their name due to the fact that they can ‘amplify’ either voltage, current or power of a signal. 

They take a weak signal and amplify it to make it more powerful.

Due to their high input impedance, MOSFETs are a great choice for creating small signal linear amplifiers. In order to perform like an amplifier, it needs to operate in the saturation region. 

The conductive region with the MOSFET can be increased or decreased by varying the gate voltage.

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