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NMOS with negative Vds

  1. Jan 16, 2014 #1
    Hello. We learn that an NMOS is in the triode region for Vds < Vgs - Vth, and in this case the drain current has a given expression. My question is, is this also valid for Vds < 0 ? What happens in this case?
  2. jcsd
  3. Jan 16, 2014 #2
    A MOSFET's channel is bidirectional so it will conduct equally well in both directions as long as the Vgs turn-on voltage is maintained. From a physics point of view either end of the MOSFET can act as the source, it just depends upon the relative voltages. The reason they are normally only connected one way is to avoid conduction in the substrate diode which has the anode connected to the source and the cathode connected to the drain in a N-MOSFET. As long as you don't forward bias that diode the MOSFET works in both directions.
    In short
    Almost all discrete MOSFET has built-in parasitic diode (body diode with cathode on the drain, anode on the source for N-channel). This diode will not conduct until drain voltage will be lower then the source voltage. If Vds > -0.6V body diode is OFF. So until the diode is OFF you can swap the drain and source.
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