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I have searched about this, and wherever I see they mention two things:

  1. How [the transistor] will be biasing
  2. Q-point must be at the cut-off and in saturation.

But I want to know in the case of the "ON" condition of a switch, for a transistor, we need to make the emitter junction forward-biased and collector junction must be forward-biased; what actually happens inside? How it will be equal to an "on" switch?

When it serves as a switch for both "on" and "off," it has different biasing. How does it internally arrange this biasing, and gives this application as a switch?

Anyone here with in-depth knowledge and answers will be a great help.

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  • \$\begingroup\$ Do you know that they are talking about a voltage/current controlled on/off device and not actually a switch that you can press? \$\endgroup\$ – Andy aka May 11 '20 at 15:55
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    \$\begingroup\$ Also, do you know how diodes work? The inner phenomenon is similar to that of a transistor just with another layer of a semiconductive material based layer. \$\endgroup\$ – user103380 May 11 '20 at 15:59
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    \$\begingroup\$ A "Microelectronics" book is where you go for "in depth" knowledge. Writing out a few chapters of a book on this site is probably beyond reasonable expectation. Short of that, there are a variety of simplifying ideas and I doubt any two of us hold the exact same simplification in mind, as quite a variety of them work "well enough" for simple use. But one way is to imagine is this: \$\endgroup\$ – jonk May 11 '20 at 16:08
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    \$\begingroup\$ If you are arranging to supply "just enough" base current for recombination needs (think of this as cleaning out the base from the accumulation of clutter that would otherwise become a barrier) but not more than that, then the collector "acts like" a current source or sink. But if you arrange to supply an overwhelming amount, far more than is actually needed, then the collector pulls itself very close to the emitter's voltage and "acts like" a voltage source, instead. It is this second behavior that is very much like a switch, as collector and emitter are now very close to each other. \$\endgroup\$ – jonk May 11 '20 at 16:08
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    \$\begingroup\$ As far as "what really happens," no one on Earth actually knows. We don't know what reality is. We have models that work really well. (Atomic model, standard model, Drude model, etc.) But no one has proven even that atoms exist. Only that when we compose ideas "as if" the atomic model is "real" then we get results (for the region of our imagination where we believe the model applies) which are congruent with experiment, interpreted through the light of other, more prosaic ideas. If you ever feel you know what reality is, then you know you're experiencing a delusion. \$\endgroup\$ – jonk May 11 '20 at 16:14
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If you want the basic gist of how it works, then imagine a transfer of electrons from one material to another. I won't talk about any equations since that would take all day.

Remember valence electrons in your chemistry class? When you apply voltage to this semiconductive material, holes will be created and whatever material around it will donate their valence electrons to those holes created.

User @jonk mentioned the "Microelectronics" book by Sedra and Smith. Well, here are some diagrams from that book.


This is known as an "n-type" where it "donates" an electron to surrounding silicon atoms.

enter image description here


This is known as a "p-type" where it "accepts" an electron from surround silicon atoms.

enter image description here


Placing these +5 or +3 atoms are known as "doping". The +4 atoms shown are silicon, which can either accept or donate their valence electrons because they have four electrons on their outer shell. There are other "+4" atoms like Germanium that is also used in transistors. The "+5" atom is shown to be on the right neighboring column of the Periodic Table of silicon and germanium such as arsenic or antimony. The "+3" atom is on the left neighboring column of the Periodic Table of silicon such as boron or gallium. The elements that I have told you about are typically used for solid state devices.

Moving on...


So when you apply voltage to this cluster of atoms, you create holes for electrons to move through.

enter image description here

Creating these holes creates conductive activity where there electrons or holes migrate from a higher concentration to a lower concentrations... THIS IS KNOWN AS DIFFUSION.

enter image description here


I know what you're asking: "But what is this threshold between conductive activity and insulate... (insulative?) activity? AKA ON and OFF respectively?"

I'm getting there. Now I have to talk about the depletion region. It's a little tricky to explain what this is but think of it as an area where holes and electrons come together like bread and butter. When you combine both n-type and p-types into a single "brick", it's known as a p-n junction.

This is what it looks like before you apply voltage (known as "equilibrium mode"):

enter image description here

and this is what happens when you apply either a positive or negative voltage (known as biasing):

enter image description here


Okay so we're finally finished talking about all of that so I won't explain exactly how this donation and acceptance of electrons will work in a transistor but just know that this is what a npn and pnp transistor look like:

enter image description here

and for an example, this is what happens when you apply some voltage to an npn: enter image description here

NPN and PNP transistors are known as bipolar junction transistors, or BJT for short. There is a different type of classification of transistors known as FETs (field effect transistor, which is unipolar).

enter image description here


But I hope that explains everything you need to know about how transistors work on a very basic level. From here you should see how a transistor is "activated".

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  • \$\begingroup\$ first a fall thanks sir for your time and work you have shown on this answer. But I know diffusion and depletion and how the transistor is made using doping, but I wanted to know what happens inside that hole and electrons movement in case of the transistor as a switch. and as in one of the comments, one mam mentioned about(@Andy aka) electronic switch, how transistor works this way??? \$\endgroup\$ – Niharika May 12 '20 at 4:15
  • \$\begingroup\$ @Niharika You're welcome :) The answer that I have provide shows what happens. When you apply voltage to a p-type region, it creates an electric field that attracts electrons from the n-type regions. \$\endgroup\$ – user103380 May 12 '20 at 19:24

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