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  1. #1
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    idss fet

    Hello,
    What is the meaning of Idss (Zero-Gate Voltage Drain Current) of a FET. Is it the smaller the value, the better the FET?

    Also what is Vgs, the smaller the better?

    Thanks & best regards.

    •   Alt11th April 2005, 04:33

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    fet parameters

    Idss ia a current when Vgs=0V.
    Value is just different for different FETs and do not descibe the transistors quality ..
    With Vgs you probably mean Vgsoff which is a Vgs voltage at which Id=0 (or near to). Again, different value for diffrent transistor with no preferences ..


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    •   Alt11th April 2005, 04:45

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    fet idss

    Thanks! Ian.

    Still I didn't get much from what you said. For example Idss (as you mentioned when Vgs=0V). In this case, the FET is off, right? Then an ideal FET should have Idss=0?



    •   Alt11th April 2005, 05:10

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    idss in fet

    At Vgs=0 FETs conduct certain current: for example the popular BF245C has Idss=12mA, but BF245A has only Idss=2mA.
    As I mentioned befor FET is off at Vgsoff what can be for N-FET somewhere below -5V and varies fro differen FETs.


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    fet parameterts

    saturn,

    A depletion mode junction FET is ON with Vgs = 0. Under this condition, the drain current is relatively independent of the Drain-Source voltage.
    This is the Idss.
    Idss is not a measure of quality. It is a measure of the maximum current that you can expect the FET to support.

    Vgs, or Vgsoff is the magnitude of Gate-to-Source voltage that is required to turn the FET OFF. It is given for a maximum Drain current, e.g., 200uA, and a specified Drain-to-Source voltage, e.g., 10V. For an N-channel depletion mode junction FET, Vgsoff is negative; for a P-channel FET, it is positive. Vgs is not a measure of quality.

    For a given family of FETs, the higher the Vgsoff is, the higher will be the Idss.

    For a given family of FETs, the higher the Idss value, the lower will be the rdson (drain-to source resistance). The rdson parameter is usually only specified for FETS that are intended for switching applications.

    Regards,
    Kral


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