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A JFET vs MOSFET discussion.

  1. Aug 21, 2010 #1
    I am a beginner with respect to MOSFETs and have just begun studying them at college 2 weeks back.

    What I am actually struggling with is the basic functional differences between MOSFETs and JFETs. Two of them are- dielectric and substrate, both of which are there in MOSFETs.

    However, I don't understand much, the significance of having a dielectric, when I thought the direct application of a gate bias could have helped, though with opposite polarity.

    Moreover, I don't understand why the term "junction" is used to describe the interface between Gate and Drain/Source?
    And why do books talk about reverse/forward biasing these "junctions" when we know they are nothing but tiny overlaps of Gate over Drain/Source?
    Do these tiny overlaps play such important role in MOSFET functioning or does it have something to do with the Substrate-Drain/Source junctions?
    And, can't a substrate bias alone work without gate bias, as in the case of JFETs, where we have only one single voltage(gate) controlling the channel as input terminal?
    Does a gate bias also control the substrate polarity, as the substrate bias does, but on the opposite face of the substrate?

    These are a few initial doubts that I wanted to share. They might appear really stupid, but for me I know they're vital. If not cleared, they could snowball into a big trouble for me in the future.

    So kindly reply...

    This is not a homework question. Please don't mistake it for one!
    Last edited: Aug 21, 2010
  2. jcsd
  3. Aug 21, 2010 #2
    The https://secure.wikimedia.org/wikipedia/en/wiki/JFET" [Broken] does not, so there is no "j" in the name (acronym). The mosfet operates on an electric field that propagates across a very thin oxide layer and thus enhances or depletes the charge carrier concentration on the other side.

    The JFET also enhances or depletes the charge carrier concentration in the same way that a https://secure.wikimedia.org/wikipedia/en/wiki/Diode" [Broken] operates. It does not have an insulating oxide layer.

    Last edited by a moderator: May 4, 2017
  4. Aug 22, 2010 #3


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    As I understand it, a JFET is distinguished still from a BJT (bipolar junction transistor). While both have pn junctions, the JFET is voltage-controlled, and requires much less input current, than the current-controlled BJT.
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