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  • 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.

In 1949 it took ENIAC computer 70 hours

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B2 中高級 美國腔

電晶體工作原理(Working of Transistors | MOSFET)

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    OolongCha 發佈於 2021 年 03 月 03 日
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