How Does a MOSFET Work?

Transistors are the basic building  blocks of modern day electronics. There are billions of transistors in the  device which you are using to watch this video, mostly of which comprises  field effect transistors. In today's video we will be exploring one  of the field effect transistor which is metal oxide semiconductor field  effect transistor or MOSFET in short.

From the basics of current and semiconductors  to the regions and structure of MOSFET and then it's complete working at last we  will also look at its circuit symbols. Before everything let us understand the flow of  current and electrons with this basic circuit. In this circuit, the current flows from  the positive terminal to the negative terminal of the battery.

Also, the electric field flows from the positive terminal to the  negative terminal outside the battery. The flow of electrons is opposite to the flow  of the electric field or current, that is, electrons flow from the negative  terminal to the positive terminal. A MOSFET is made from a semiconductor  material such as silicon.

Semiconductors have conductivity  between conductors and insulator. Hence, to make a semiconductor a good conductor  we introduce impurities in the pure crystal. There are two types of impurities that are added.

If the impurities are pentavalent, then  the resulting semiconductor is n-type. In n-type electrons are the  majority of charge carriers. And, If the impurities are trivalent then  the resulting semiconductor is p-type.

In p-type holes are the  majority of charge carriers. Now if we join them then at the junction the  electrons from n-type will fill the holes in p-type depleting the charges near the junction. This region is known as the depletion region.

If we connect p-type to the positive terminal and n-type to the negative terminal of the battery, then the depletion layer reduces and this is called forward bias. If the polarity of the battery is reversed the depletion layer increases, and this is called reverse bias. MOSFETs are of two types, enhancement type and depletion type.

Both of the types are further divided into two types, N - channel and P - channel. In this video, we will look at N - channel MOSFET only. First, let's see the N-channel enhancement type MOSFET.

This is the structure of MOSFET. The yellow regions are n-type semiconductors  and blue are p-type semiconductors. This terminal is connected to the substrate or body hence it's called the substrate or body terminal.

These two terminals are called source and drain terminals. Between these terminals we have a thin layer of insulator or dielectric. Above this insulator another terminal is attached,  It is called the gate terminal.

These are all four terminals of MOSFET. As the MOSFET is symmetrical, that is, the  source or the drain can be interchanged. Hence, the source terminal and substrate  terminal are connected internally, and the MOSFET we see has three terminals.

Also, this stops any current flow from the substrate to the source as they are at the same potential. Now, in the MOSFET we want to flow  conventional current from drain to source. So let's connect a battery  between drain and source.

This voltage is called Vds, as it's  between the drain and the source. Also, we can see the graph  of drain current versus Vds. The positive end of the battery increases  the potential at the drain terminal, thus increasing the depletion region  between the drain and substrate.

Due to this there will be no current flow from  the drain to the source and the MOSFET is off. This is also called the cutoff region. Now, to flow current from drain to source we have to create a channel between them.

To create the channel we connect a small voltage source between the gate and substrate  with the positive terminal to the gate. This voltage is called Vgs, as it's  between the gate and the source. The substrate is a p-type semiconductor.

Hence, the charge carriers are holes. But, there exists some free electrons  as minority charge carriers. The battery creates an electric  field inside the substrate.

Due to this field the electrons in  the substrate flow opposite to the electric field, that is, towards the gate. Due to the presence of an insulator these electrons cannot flow from the substrate to the gate. And thus they accumulate near  the gate in the substrate.

We know that a capacitor is used to  store charge on two metal plates. Also, we can increase the capacitance by placing  an insulator or dielectric between the plates. Similarly, the insulator or dielectric in MOSFET, not only blocks the electrons but  also increases the charge on them, thus attracting more electrons.

Now, If we increase Vgs more electrons get attracted towards the gate and these  electrons start filling some of the holes. Also, due to the stronger positive charge of  gate holes start moving away from the gate. Now, in this region we can see there  are no holes but we have free electrons.

Due to these electrons, the region near the gate  becomes negative or an n-type semiconductor. This creates a channel that connects the source  and the drain with each other internally. In effect we have created a pipe between  the source and drain so that electrons can move from the source to the drain.

The thickness of the channel can be controlled by changing the gate voltage. As the voltage increases or decreases the width of the channel also increases or decreases respectively, And the voltage at which the channel is  formed is called the threshold voltage. As this channel is created now we can flow current  from the drain to the source by the channel.

The flow of conventional  current is from drain to source, but the flow of electrons is opposite  to it, that is, from source to drain. This is also the reason why they  are called the source and the drain, because the source supplies  electrons to the channel and the drain collects the  electrons from the channel. As the conventional current flows from  drain to source it is called drain current.

Now the MOSFET is in the ohmic or linear region. In this region, it follows ohm's law, that is, as the voltage increases the  current increases linearly. But, as the voltage increases the depletion  region between the drain and substrate will increase as they are reverse biased.

Also, the channel begins to deplete towards the drain end. This is because, the drain is at a positive potential and negative charges from the channel closest to the drain are being pulled into the drain. This reduces the width of the channel restricting the flow of charges  and reducing the flow of current.

As we increase the voltage, a  point will be reached when the channel is completely pinched off. This is called the pinch-off effect. In real cases the channel is  not completely pinched off.

Due to the large flow of electrons in the channel,  a number of electrons will keep the channel. Hence, instead of stopping the current  there is a constant saturated current. This current is called saturation  current and the voltage at which it occurs is called saturation voltage.

Now, if you increase the voltage the reverse biasing will increase and the channel will further decrease but the current will not increase because it's saturated. This is known as the saturation region. But if you still want to increase the  current in the MOSFET, how will you do it?

(THINKING) Now that you thought about it  let us see it in animation. As you may have remembered, the width of  the channel is controlled by the gate. Hence, by increasing the gate voltage we  can increase the width of the channel.

Now, if we increase the voltage  the current will also increase. Again at a certain voltage pinch off  will occur and saturation will occur. Then again increase the gate voltage.

MOSFETs are also called voltage control devices,  because the amount of voltage at the gate controls the flow of current from drain to source. Also, there is no current flow from the gate. If you have noticed the two graphs that we created are the characteristics of the MOSFETs.

This is the drain characteristics and this is  the transfer characteristics(constant Vds). There is another type of MOSFET  called the depletion type. This is similar to the enhancement type except  the channel is also formed while doping.

That is, the channel that was formed by the gate voltage in enhancement type is present by default in depletion type. All other working principles are the same except that, the depletion type requires a negative gate voltage to turn off it is normally on (Normally closed) while the  enhancement type is normally off (Normally open). Now we know how a MOSFET works let  us look at its circuit symbols.

There are four terminals, source,  gate, drain, and substrate. The gate is not directly connected to  the substrate an insulator is present. Hence, the gate terminal  is floating in the symbol.

Source and substrate are connected  internally, hence, we connect the source and substrate in the symbol also. If the lines are broken then its enhancement type as the channel needs to be formed, else if it's one solid line then the channel is present and its depletion type. If the arrow is pointing towards the substrate then its N channel or the electrons flow towards the gate to form an N channel.

Otherwise its P channel or the electrons  flow away from the gate to form a P channel. These MOSFETs are present in almost all electronic  devices and they need electricity to work. Watch this video on how the mobile charger works  or continue by watching this video on how mobile knows about its orientation.

Thank you for watching.