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Mosfets in on state

  1. Jul 16, 2011 #1
    hello everyone,
    pls tell me "how can a mosfet be in on state even if no current os passing through it?"
    can it be explained by the fact that channel is induced by Vgs & current passes when Vds>0.
    so Vgs turns mosfet in on state and current is zero because Vds would be zero./
    Thanks in advance.
  2. jcsd
  3. Jul 16, 2011 #2
    I don't understand the question very well.

    MOSFETs come in 2 flavors. They can be manufactured as depletion or enhancement devices.

    Enhancement MOSFET transistors are transistors that can only operate when certain Vgs is applied(Positive for n-MOSFET). If you by "it" mean gate, current doesn't flow through gate(approximation). They are called "Enhancement" because channel is "enhanced" through Vgs. Vgs must be sufficient enough for the channel to start forming. Further raising of Vgs after threshold voltage will only increase conductivity of the channel. Applying Drain-Source voltage would cause current to flow.

    Depletion MOSFET transistors are transistors that can operate even with Vgs is zero. They have an implemented n channel which can be squeezed by applying negative voltage and enhanced further by applying positive voltage.

    I think MOSFET is biased as "on" when current IS passing through it. But I could be wrong.

    Can you explain a bit more what you are asking?
    Last edited: Jul 16, 2011
  4. Jul 16, 2011 #3


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    I don't understand the question either. To me it is like a question that is similar to how one wonders how a mechanical switch can actually be 'on' when its contacts are closed but not actually passing current because for instance something else is interupting the current.
  5. Jul 16, 2011 #4
    yes your statement completely describes nature of mosfet,i guess.
    i just read a statement that "mosfet can be in on state even if no current is passing through them."
    n i think i have understood now since Vgs only induces the channel(kind of controls resistance of the channel) and Vds initiates current flow.lower the resistance,higher the current.

    n by 'it' i meant mosfet.
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