Working of Transistors | MOSFET

Working of Transistors | MOSFET

Alyson Nader Articles, Blog


In 1949 it took ENIAC computer 70 hours
to calculate the value of Pi at 2,037 digits. Now the smartphone in to my 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 call MOSFET. Lets get into a 3D animation
to run the working of a MOSFET. 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 it’s pure form a semiconductor
has very low electrical conductivity. However, when you introduce
a controlled amount of impurities into the semiconductor material
it’s 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 silicone does not
have any free electrons and because of this
it’s 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 impurities 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 lets 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 N-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 flow. 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 it’s 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 and the alarm. MOSFETs open 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 text 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 being 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!

100 thoughts to “Working of Transistors | MOSFET”

  6. Very good and clear presentation.We all need more videos (with the same method) for other phenomena. Thank you

    Reply

  7. For fuck sake can you get a English person to talk English so it's easier to understand. Oh so they do have some sense.

    Reply

  10. You have really sparked my interest in engineering.
    Thanks a ton man.
    Please please please upload more videos.
    Best wishes

    Reply

  17. What I don't understand is is we are in a battery revolution trying to make more efficient and powerful batteries but yet we have the capabilities today we could use the process that we used to make transistors into very small micron battery cells and just later them and layer then and we could have a thousand if not billions and trillions of cells in the size of a lithium battery

    Reply

  21. This Is AWESOME. I hope you make many many more videos, longer ones, more in depth.

    Your videos are perfect for someone who already knows stuff on this topic, but no clarity.

    Reply

  31. I'm really grateful for this channel that they are using the power of animation for something that is so direly needed in this world.
    Thank you.

    Reply

  34. Imagine the trillions of transistors and trillions of switching required to render and serve and display this content

    Reply

  41. Nice video with a excellent animation. Please make more of these videos on scr and other power electronics components. Thanks.

    Reply

  43. WOWWOWOW…That was just awesome. hE EXplained the topic so easily even it is going to be in mind for next generation.

    Reply

  61. Hi sir…..make a animation video in 555 timer ic….or also make a animation video that how to electron go to in circuit……..

    Reply

  69. This video is excellent. I've been struggling to find a study material which explained the role of dielectric in gate but i couldn't find it in any video. This is perfect and extremely clear, things we would never find in a textbook. Thank you!

    Reply

  77. Amazing video!

    Question: What does "less noisy" means? You mentionned this at the end when you pointed out that there is always 1 type of carrier: electron or hole.

    Reply

  85. 3:34 Sir We cannot apply DC source to a capacitor because Capacitor block DC and only allows AC voltage…

    Reply

  91. Bro does hole moving really
    No only electrons moving it may n type or p type
    U need to read hole definition again I think

    Reply

  93. Where were u when I was doing my engineering 😢😢😢…although I m millennial still way earlier than digital revolution…current college going students don't know how lucky they are as anything can b easily learned from Internet and u don't have to care abt t qualifications of t teacher which unfortunately in India is quite low and to make worse they r only interested getting salary 😥😥

    Reply

  96. Okay, I understand the AND+OR transistor logics. The key is the gate mechanism as it dictates what circuit is allowed to flow and what is not. So AND means that both input transistors gates need to be active, which in turns activates the gate of the third output transistor because of the complete circuit, which resulting in positive output signal. OR is similar, but instead of a chain of transistors they are connected parallel, so a signal on one of the two input transistors gates results in a positive signal to the output transistors gate.

    But the NOT function is confusing to me as it is presented because NOT is a inversion of the signal. As in if there is a signal to the function then the output is negative and vice versa. But on the schematic, as presented, it just looks like a regular basic MOSFET, so a signal to the gate should result in a positive output.

    Reply

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