FET operation

Discussion in 'Off-Topic' started by The Electrician, Aug 27, 2009.

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  1. The Electrician

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    What controls a MOSFET's drain current, gate voltage or gate charge?
     
  2. beenthere

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  3. alphacat

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    I believe you're referring to MOSFET's saturation region.
    In saturation region, the drain current is highly depended on VGS and little on VDS.
     
  4. The Electrician

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    The Wikipedia article says early on that "A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration by a MOS capacitance...", but later it says "The MOSFET operates like a resistor, controlled by the gate voltage...".

    It could even be said that the gate voltage is an indicator of the gate charge.

    Something like that was said in the thread from which you moved my post. The discussion over there had become once again the philosophical discussion it had become in previous threads. My question was intended to address the interpretation aspect of the discussion by bringing in an analogy to another type of semiconductor.

    I'll re-post with reference to the original thread in the off-topic forum.
     
  5. rjenkins

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    The device specifications are based on voltage, not charge.

    Charge is proportional to the applied voltage and gate capacitance. As that capacitance is probably trivial compared to the external circuit capacitance, for most purposes it can be ignored.

    The only time the capacitance matters is if you are trying to drive it at high frequency, then that is a straight calculation based on total gate circuit capacitance to get the desired rate of voltage change, the 'charge' again is irrelevent.
     
  6. alphacat

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    Its not just the Gate voltage that matters.
    In order to have free electrons in the MOSFET's n-channel, the Gate-Source voltage should be higher than the threshold voltage.

    So even that the usual VTn is around 1V-2V, you could have VG equal 5V, but still have the Mosfet in cut-off if the Source's voltage is not lower than VG - VTn.
     
  7. The Electrician

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    I assumed that everyone would understand that when I said gate voltage I meant with respect to the source; Vgs in other words.

    I started this thread anew in the off-topic forum.
     
  8. beenthere

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