Basic MOSFET diode-connected question

Discussion in 'General Electronics Chat' started by newbie217, Sep 3, 2009.

  1. newbie217

    Thread Starter Active Member

    Apr 12, 2009
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    Hello,

    I'm trying to familiarize myself w/ some basics of MOSFET operation + using SPICE to do some easy simulations. (Please see attached image of circuit I am trying to construct)

    I ran into the following problem. First, I biased a MOSFET by diode connecting it, drain tied to gate. I then set the source to ground. This means that the MOSFET is operating in the active region by the following: VDS > VGS - VT. I see a drain current, which is what I expect to see. So everything is ok.

    Next, I tried to make some slightly different connections to confirm that the MOSFET would also operate in the cutoff region. However, when I tied the Source to the Gate, I still see appreciable current flow. In this new case, Gate = Source, so VGS = 0, and the MOSFET should have no drain current. In theory, there should be no conductive channel that forms from source to drain, right? In simulation, I am seeing a current, and a voltage drop across the drain resistor. I'm trying to figure out what is going on here. Am I missing some key points?

    Thanks for the help!
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    In the case with gate connected to source, you need to swap +5V and ground.
     
  4. newbie217

    Thread Starter Active Member

    Apr 12, 2009
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    I know that this transistor is hooked up incorrectly. My question is when I simulate it, why does it show such a large drain current? Shouldn't I see close to 0 A since VGS = 0, and the NMOS is in cutoff?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Jony130 explained it. The body diode is between source and drain, anode to source. You are seeing current through that diode, not through the channel, which is indeed cut off.
     
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