JFET Structures

Discussion in 'Homework Help' started by AD633, Jun 22, 2013.

  1. AD633

    Thread Starter Member

    Jun 22, 2013

    I am trying to understand how a JFET(junction field effect transistor works).
    In the attached picture.
    I know that a JFET is a basically a channel of semiconductor material, doped to contain an abundance of positive charge carriers or holes (p-type), or of negative carriers or electrons (n-type) and the flow of electric charge through a JFET is controlled by constricting the current-carrying channel but i do not understand the following points:

    In what consist and how appear the depletion regions and why does the depletion region does not have uniform width?

    In which case the closure of the channel occur?(Id=0 A)

    Last edited: Jun 22, 2013
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    I'm unable to open your diagram. Please add a diagram in PGN file.
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Ok I see you diagram now. But I'm unable to help you. Why? Well because despite the fact that I understand how JFET "work" and I can use it in a real word circuit. But I never try to full "understand" semiconductor physics, because in real word we don't need to understand it (unless you are actually working with IC design). But maybe someone else can help you with your problem.
  4. circuitfella11


    May 10, 2013
  5. Shagas

    Active Member

    May 13, 2013
    As Jony130 said , if you are starting to learn about JFETs then it's much more important to learn how it behaves, their electrical properties and how to use them in circuits .
    Concerning the physics , I think it's enough to know that the JFET acts like a hose and By applying a voltage between the high input impedance base and the source you are pinching the hose and thereby restricting flow
  6. tshuck

    Well-Known Member

    Oct 18, 2012
    I would tend to disagree with the above comments regarding a high-level operation of the JFET; I feel that I know how a JFET works because I know why a JFET works, but that could just be me.

    The depletion region is misshapen because of the voltage impressed on its terminals: V_{DD}. If, however, V_{DD} = 0V, then the channel will be of uniform thickness, provided the layout is as shown.

    Instead of reiterating the content, the ebook has summarized the operation of the JFET quite nicely.