In 1949 it took ENIAC computer 70 hours
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.