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I can't find the definition of unipolar and bipolar junction in a transistor, for example BJTs has a bipolar junction and MOS-FET has a unipolar junction, what does it mean unipolar and bipolar junction?

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  • \$\begingroup\$ NPN and PNP transistors each have two PN junctions. That's why they are bipolar. A FET in contrary only has a N or P doted channel, and one gate or body diode. That's only one junction. \$\endgroup\$
    – Janka
    Feb 25, 2018 at 9:24
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    \$\begingroup\$ @Janka A standard MOSFET has 2 PN junctions as well. \$\endgroup\$
    – Matt
    Feb 25, 2018 at 18:55
  • \$\begingroup\$ Way back when dinosaurs roamed the Earth there was another kind of semiconductor device called a point-junction or more accurately point-contact transistor. This is possibly the only kind of transistor one could fabricate with hand tools. \$\endgroup\$
    – Dan1138
    Sep 21, 2018 at 3:39

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It doesn't have anything to do with the junction. The terminology stems from the charge carriers involving current flow in the device. BJT is called bipolar because the current in a BJT flows due to both electron and hole carriers, whereas the current in a Field Effect Transistor (FET) flows due to either electron carriers for N-type FET or hole carriers for P-type FET, hence they're called unipolar transistors.

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  • \$\begingroup\$ I had the impression that etymology of bipolar was invented after FETs had become more widespread. The old difference was about bipolar transistor vs. unijunction Transistor. \$\endgroup\$
    – Janka
    Feb 25, 2018 at 9:45
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No, you're reading it wrong. Both 'Bipolar' and 'junction' are not "connected"... I mean to say it is not like junction is bipolar or unipolar. Actually 'Bipolar' means current conduction in device is due to both polarity charge carriers, electrons and holes. In NPN BJT, electrons are majority charge carriers and holes are minority charge carriers but holes still play their (small) part in conduction hence called bipolar transistor. FETs in which only one, either electrons or hole plays part in current conduction are called 'Unipolar' transistors

I haven't heard "Unipolar junction" term but if you are referring to UJTs then 'UniJunction' word signifies that the device has only one P-N junction and not that the current conduction is due to only one type of charge carriers, however that's true.

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