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Transistors, How do they work ?

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Jul 14, 2016

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Transistors, How do they work ?
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  • The invention of transistors revolutionized human civilization like no other technology
  • At the heart of a smartphone lies a processor
  • And this processor holds about 2 billion transistors
  • What do these incredibly tiny devices do?
  • How do they work?
  • Transistors can act like a switch with no moving parts
  • They can amplify a weak signal, in fact amplification is the basic function of a transistor
  • First let's understand the basis of transistors, we will come back to the application part later
  • Transistors are made of semiconductors such as silicon
  • Each silicon atom is bonded with four neighboring silicon atoms
  • Silicon has four electrons in its valence shell
  • Let's replace the silicon atom with a four-handed smiley
  • Each hand holds one electron
  • Each one of these electrons goes for sharing with a neighboring silicon atom
  • This is known as a covalent bond
  • Currently the electrons are in their valence band
  • If the pure silicon has to conduct electricity
  • The electrons have to absorb some energy and become free electrons
  • Thus the pure silicon will have a low electrical conductivity
  • A technique called doping is used to improve the conductivity of semiconductors
  • For example say you inject phosphorus with five valence electrons
  • Here one electron will be free to move in the system
  • This is known as N-type doping
  • On the other hand if you inject boron with three valence electrons
  • There will be a vacant position for an electron this vacant position is known
  • as a hole and a neighboring electron can fill this hole at any time
  • This electron movement is visualized as holes moving in opposite direction
  • We call this P-type doping
  • If you dope a silicon wafer in the following manner a transistor is born
  • But if you really want to understand how a transistor works,
  • we have to get a clear idea of what happens at the electron level of a more basic component, a diode
  • A diode is formed when you dope one part of silicon as a P-type and the other part as an N-type
  • Something very interesting happens at the boundary of the N and P joint
  • The abundant electrons on the inside
  • will have a natural tendency to migrate to the holes that are available on the P side
  • This will make the P side boundary slightly negatively charged
  • and the N side slightly positively charged
  • The resulting electric field will oppose any further natural migration of the electrons
  • Ff you apply an external power source across the diode as shown,
  • the power source will attract the electrons and holes
  • Electricity flow is impossible in this case
  • However, if you reverse the power connection the situation is quite different
  • Assume that the power source has enough voltage to overcome the potential barrier
  • You can immediately see that the electrons will be pushed away by the negative terminal
  • When the electrons cross the potential barrier,
  • they will be drained of energy and will easily occupy the holes in the P region
  • But due to the attraction of the positive terminal,
  • these electrons can now jump to the holes nearby in the P region
  • and flow through the external circuit
  • This is known as the forward biasing of a diode
  • Just keep this simple principle of a diode in mind,
  • you will understand the operation of a transistor very easily
  • Now back to the transistor
  • Note that the P layer is really thin and lightly doped
  • You can easily see that a transistor is essentially two diodes sandwiched back to back
  • So whichever way you connect the power source,
  • one diode will always be reverse biased and block the electricity flow
  • This means the transistor is in the off state
  • Now let's connect a second power supply as shown
  • The power supply should have enough voltage to overcome the potential barrier
  • So this is just a forward biased diode
  • Thus a high number of electrons will be emitted from the N region
  • Just like in a diode a few electrons will combine with the holes
  • and jump across the neighboring holes and flow to the base
  • But there are a lot more electrons that has crossed to the P side,
  • what will these remaining electrons do?
  • Think for a moment
  • The remaining electrons will get attracted by the positive terminal of the first power source
  • and will flow straight as shown
  • Note that the P region is very narrow
  • which ensures that no remaining electrons flow to the positive terminal of the second power source
  • In short a small base current is amplified to a high collector current
  • You can easily correlate the naming of the transistor terminal with the nature of electron flow
  • Ff you can increase the base current the collector current will increase proportionally,
  • this is a clear case of current amplification
  • The kind of transistor we have discussed is called a bipolar junction transistor
  • Let's replace this representative transistor with a realistic one
  • You can further improve the amplification by introducing one more transistor
  • Yhe base of this transistor is connected with the emitter of the first transistor
  • Ff you introduce a weak fluctuating signal at the input like what you would find in a microphone,
  • you will get an amplified signal at the loudspeaker
  • The other interesting thing you can note about this basic circuit,
  • is that depending on the value of the applied voltage,
  • the transistor can be either on or off
  • Here the transistor acts as a switch
  • This property of the transistor
  • opens the doors to the world of digital electronics and digital memory
  • Using two BJT's you can build the basic dynamic memory element of computer:
  • a flip-flop
  • To know more about it please watch the video on the transistor as a switch
  • Please support us at patreon.com so that we can continue our free educational service
  • Thank you!

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Dear friends, Please support us at Patreon, so that we can continue our free educational service
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The invention of transistors revolutionized human civilization like no other technology. This video demonstrates working of a Bipolar Junction Transistor (BJT) with it's practical applications such as transistor as an amplifier and as a switch with help of animation. Along with transistor working of diode is also explained in the video. The video covers following topics - structure of Silicon atom, doping, N type doping, P type doping, working of Diode, working of NPN transistor and dual stage amplification.

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