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Transistors – Field Effect and Bipolar Transistors:  MOSFETS and BJTs

Transistors – Field Effect and Bipolar Transistors: MOSFETS and BJTs


The red spheres represent positive current, which is in the opposite direction of electron current. In a “Bipolar Junction Transistor”, a small current flowing into the middle terminal controls a much larger current through the transistor’s two other terminals. Bipolar Transistors are one of the two main types of transistors. In this other type of transistor, the middle terminal does not have any current passing through it during steady state conditions. Here, it is the “voltage” of the middle terminal that controls the current through the other two terminals. This is called a “Field Effect Transistor.” There are two main types of “Field Effect Transistors” and two main types of “Bipolar Transistors.” We can tell them apart by whether the arrow is pointing toward or away from the middle terminal. In bipolar transistors, the positive current flows in the direction of the arrow. The red spheres symbolize the positive current. This is a “NPN” bipolar transistor. This is a “PNP” bipolar transistor. Here, it is the positive current flowing “out” of the middle terminal that controls the transistor. Let’s now focus on Field Effect Transistors. The two main types of “Field Effect transistors” are two different types of “Metal Oxide Semiconductor Field Effect Transistors”, abbreviated “MOSFETs.” This is an “N Channel” MOSFET. As always, the red spheres represent the direction of positive current. Here, a decrease in the voltage on the middle terminal will cause a decrease in current. An increase in the voltage on the middle terminal will cause an increase in current. This relationship is reversed for a “P Channel” MOSFET. In both cases, the voltage that controls the transistor is the voltage between the terminals that we call the “Gate” and the “Source.” In the N channel example, the “Source” is shown at the bottom. In the P channel example, the “Source” is shown at the top. In both cases, the voltage difference between the “Gate” and the “Source” has to exceed a certain “threshold voltage” before it starts having any effect. Since it is the voltage between the “Gate” and the “Source” that controls the transistor, the current will not increase much if we increase the voltage at the “Drain.” But, if we reduce the “Drain” voltage, we will eventually reach a point where the current will significantly decrease. The behavior of Bipolar Transistors is different than Field Effect Transistors, and the terminals have different names. Here, when the current is flowing, the difference between the “base” voltage and the “emitter” voltage stays at about 0.7 volts, and the “collector” current is the “base” current multiplied by a large constant number. Suppose we add another resistor as shown. A change in the current through this resistor causes a change in the voltage drop across it. Here, the collector voltage is 0.3 volts above the emitter voltage. If the base current is increased further, the collector voltage is unable to go any lower. Here, the all the currents are staying constant. Here, the bipolar transistor is behaving like a switch in the on position, but it is not an ideal switch because there is a 0.3 volt drop across it. On the other hand, when a “MOSFET” behaves like a switch in the on position, it is not an ideal switch because it behaves like a resistor with a small resistance value. A MOSFET is unable to block in the reverse direction because there is a diode inherently built into its body. Also inherently built into each MOSFET are capacitors between each of the terminals. Although it is not shown in this video, these capacitors need to charge and discharge, and there is therefore a brief momentary current through the gate terminal when the gate voltage changes.

93 comments on “Transistors – Field Effect and Bipolar Transistors: MOSFETS and BJTs

  1. I've never been in a classroom where the lecturer switches on a music player before he starts talking. Why is it that every YouTuber thinks what they say will be more understandable, or make a greater impact if there is music playing?

  2. Love these videos. Are there versions of these without the music, or at least with the music playing much more softly?

  3. You can help translate this video by adding subtitles in other languages. To add a translation, click on the following link:

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  4. I'm studying to become an electrical engineer and have yet to cover transistors. They still seem like black magic to me.

  5. Just curious: is one of the reasons that FETs tend to run cooler than BJTs is that current does not flow through the gate? (as opposed to the base)

  6. Very very nice video! got everything i wanted, and liked it so much i added portuguese subtitles! thank you Khutoryansky, just got a new sub.

  7. Were da pluck are the represented electrons, do the red balls represent the hole theory in semiconductors. Ya not explainin the bias on these semiconductors and how they cause current flow.

  8. Hi there. I just wanted to say thank you for the animation because this is really helpful for me to understand these devices better than the drawings in my textbook.

  9. Awesome videos!!. I would like to see some other electronic topics like output transformers, triode, pentode, cathode, anode, plate, grid, screen, voltage drop, grid leak resisters, choke, impedance reflection…etc.. Maybe take an old schematic and map it out like these videos, showing current, voltage, amps, etc.

  10. Проблема 99 процентов специалистов в электронике, в том , что они даже представления не имеют, как работает биполярный транзистор

  11. and what is a positive current? never heard about that. for the fact that u dont say much your video is very long. music makes no sense by the way.

  12. Man…. I love these videos! Thank you so much for taking the time to make these wonderful, educational gems ❤️

  13. Vídeo nota 1000000000000.
    No transistor NPN entra 3 bolinhas no Coletror e 1 na Base, e sai 4 no Emissror.

    No PNP é o inverso, entra 4 no Emissor, Sai 1 na Base e, sai 3 no Coletoror.

  14. I can here for help with my bipolar illness. I left with knowledge to handle electrons. The brain has electrons, so I'm half way there!

    Off to continue building 🤖 🤖 🤖….

  15. To see subtitles in other languages: Click on the gear symbol under the video, then click on "subtitles." Then select the language (You may need to scroll up and down to see all the languages available).
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  16. Am I the only one concerned with the position positive current model? Current theory cites electron flow.

  17. исправить название за 2 года – походу не судьба)

  18. Love these, but using the conventional "current flows opposite of electron flow" is really confusing. Since this is a physics channel, I'd think that representing current flow as electrons would make more sense.

  19. L'excellente animation vidéo accompagne avec bonheur les explications simples et limpides. Et ça rentre tout seul. Brrraaavooo!

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