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How to simplify behavior of a field-effect transistor

  1. Feb 2, 2015 #1
    Are there simple governing equations for FETs like there are for BJTs, and if so, what are they? For example, in the BJT case we have
    IC = β×IB
    IE + IC + IB

    I'm sure it depends on the type - MOSFET, JFET, etc. - so anything anyone could contribute would be helpful. From what I understand, the source-to-drain current is proportional to the gate voltage, so if it's linear it would be something like
    IDS = β×VG

    From what I've seen Googling IV curves, that's fairly close up until saturation. Am I off base here, or am I on the right track?
     
  2. jcsd
  3. Feb 3, 2015 #2

    Baluncore

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  4. Feb 3, 2015 #3

    LvW

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    (a) BJT
    The second equation should be: IE=IB+IC.
    More than that, for the BJT you forgot the control function IC=Io*[exp(VBE/Vt)-1].
    This equation is important because the slope of this function gives the transconductance g=IC/Vt.

    (b) FET
    Here, the input-output relationship (control function) is ID=ID,max*[1-(VGS/VP)²]
    with VP: pinch-off or threshold voltage, respectively.
    Again, the transconductance is the slope of this function: g=2*SQRT(ID*ID,max)/|VP| .
     
  5. Feb 3, 2015 #4

    Svein

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    Or you just look up the characteristics in a data sheet:
    IDVGS.jpg
     
  6. Feb 4, 2015 #5

    donpacino

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    the datasheet curve only works at one VDS value, usually they give you multiple curves
     
  7. Feb 4, 2015 #6
    Thanks, everyone!
    Baluncore: Wow, I must have missed that. Thanks for pointing it out!
    LvW: Whoops, sorry about the typo. Yes, we only used the linear approximation in the class I took - we didn't need anything finer at the time. I appreciate your contribution!
    Svein and donpacino: Thanks for pointing those out! Since I didn't feel that I knew enough about how I could use one, I hadn't looked into the datasheets much, but now I know what to expect.
     
  8. Feb 5, 2015 #7

    Svein

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    Yes, but when I tried to copy from a real datasheet it was "protected". So for a real datasheet, search at On Semiconductor or Texas Instruments.
     
  9. Feb 5, 2015 #8

    LvW

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    Sorry, I have to correct one of my formulas (post 3). The expression in the outer brackets [... ] must be sqared:
    correct: ID=ID,max*[1-(VGS/VP)]²
     
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