to calculate the value Pi up to 2037 digits.

Now the smartphone in your hand

can do the same task in

0.5 seconds.

This miraculous growth in speed was made possible

by a tiny device inside the electronic gadget

called a transistor.

More specifically,

a type of transistor called MOSFET.

Let's get into a 3D animation,

to learn the workings of a MOSFET.

(soft music)

- [Instructor] MOSFET is an electronically driven switch

which allows and prevents a flow of current

without any mechanical moving parts.

Like any other conventional transistor,

a MOSFET is also made from a semiconductor material

such as Silicon.

In its pure form,

a semiconductor has very low electrical conductivity.

However, when you introduce

a controlled amount of impurities

into the semiconductor material,

its conductivity increases sharply.

This procedure of adding impurities

is called doping.

To understand the physics of doping,

let's first understand the internal structure of Silicon

and also that of the impurity known as a dopant.

Pure Silicon does not have any free electrons

and because of this its conductivity is very low.

However, when you inject an impurity

which has extra electrons into the Silicon,

the conductivity of the resultant material

increases dramatically.

This is known as n-type doping.

We can also add impurities with fewer electrons,

which will also increase the conductivity

of pure Silicon.

This is known as p-type doping.

When the concentration of the impurity is lower,

the doping is said to be low or light.

On the other hand, if it is higher,

the doping is referred to as high or heavy.

Now let's get back to the workings of MOSFETs.

If you dope a Silicon wafer

in the following manner,

you will get the basic structure of a MOSFET.

It is interesting to note that

even in the p region,

there are very few free electrons

that are capable of conducting electricity.

We call them minority carriers.

Later we will see why the minority carriers

are significant in the MOSFET.

Whenever a p-n junction is formed,

the excess electrons in the end region

have a tendency to occupy the holes

in the p region.

This means that the p-n junction boundary

naturally becomes free of holes

or free electrons.

This region is called a depletion region.

The same phenomenon also happens

in the p-n junction of the MOSFET.

Now let's connect a power cell across the MOSFET

and observe what happens.

On the right hand side p-n junction,

the electrons are attracted

to the positive side of the cell

and the holes are moved away.

In short, the depletion region width

on the right hand side

is increased due to the power source.

This means that there won't be any electron flow

through the MOSFET.

In short, with this simple arrangement,

the MOSFET will not work.

Let's see how it is possible

to have an electron flow in the MOSFET

using a simple technique.

To do this, we first need to understand

the workings of the capacitor.

Inside the capacitor,

you can see two parallel metal plates

separated by an insulator.

When you apply a DC power source across these,

the positive terminal of the cell attracts electrons

in the metal plate

and these electrons are accumulated

on the other metal plate.

This accumulation of charge

creates an electric field between the plates.

Let's replace one plate of the capacitor

with the p-type substrate of the MOSFET.

If you connect a power source

across the MOSFET as shown,

just as in a capacitor,

the electrons will leave the metal plate.

In a MOSFET, these electrons will be dispersed

into the p substrate.

The positive charge generated on the metal plate

due to the electron displacement

will generate an electric field as shown.

Remember there are some free electrons

even in the P type region.

The electric field produced by the capacitive action

will attract the electrons to the top.

We will assume the electric field generated

is quite strong

and then observe the electron flow.

To make things clear, let's rewind the animation.

Some electrons were recombined with the holes.

And you can see that the top region

becomes overcrowded with electrons

after all the holes there are filled.

Just below this region,

all the holes were filled,

but there were no free electrons either.

This region has become a new depletion region.

You can see that this process

essentially breaks the depletion region barrier

and a channel for the flow of electrons is created.

If we apply a power source

as we did at the beginning of this video,

the electrons easily flow as shown.

This is the way a MOSFET turns to the on state.

You can easily correlate

the naming of the transistor terminals

with the nature of the electron flow.

If the applied voltage is not sufficient enough,

the electric field will be weak

and there won't be a channel formation

and hence no electron flows.

Thus just by controlling the gate voltage,

we will be able to turn the MOSFET on and off.

Now let's see a real life example

where a MOSFET works as a switch.

Consider this heat based fire alarm.

The thermistor in the circuit decreases its resistance

with an increase in temperature.

Initially at room temperature,

the voltage at the gate is low

due to the high thermistor resistance.

And that is not sufficient to turn on the MOSFET.

If the temperature increases,

the thermistor's resistance decreases.

This will lead to a high gate voltage,

which then turns on the MOSFET (alarm beeping)

and the alarm.

MOSFET has opened the door to digital memory

and digital processing.

Here you can see four MOSFETs combined together

to form the basic memory element of a static Ram.

At the lowest level,

MOSFETs are interconnected to form logic Gates.

At the next level, the Gates are combined

to form processing units

that perform thousands of logical

and arithmetical operations.

Unlike BJTs, MOSFETs have a scalable nature.

So that millions of MOSFETs

can be fabricated on a single wafer.

A BJT wastes a small part of its main current

when it's switched on.

Such power wastage is not there in MOSFETS.

The other advantage of a MOSFET

is that it only operates

with one type of charge carrier,

be it a hole or an electron.

So it is less noisy.

These are the reasons why MOSFETs

are the popular choice in digital electronics.

We hope this video gave you

a clear conceptual overview

of the workings of MOSFETs,

and please don't forget to support us on Patreon.

Thank you.