What is the equation for a N-channel MOSFET's current value (\$I_{DS}\$) given a negative drain-to-source voltage \$V_{DS}\$?

I can find plenty of forward biased I-V curves for the N-MOSFET, but I haven't found any reverse biased ones.

  • \$\begingroup\$ \$V_{DS}\$ is the drain-to-source voltage, not the source-to-drain voltage. Can you edit your question to be clear which one you are asking about? \$\endgroup\$
    – The Photon
    Dec 16, 2014 at 18:30
  • \$\begingroup\$ @ThePhoton it's a N-channel, so \$V_{DS}\$ \$\endgroup\$
    – Vial
    Dec 16, 2014 at 21:45

2 Answers 2


Basic threshold model--

It is in the triode region for Vgs > Vth, regardless of the direction (and |Vds| low).

There is also a body diode in parallel so if -Vds is more than than a few hundred mV that must be taken into account as the diode will conduct.

If you want more accurate subthreshold modeling, someone has probably done it, but this suffices for many or most applications where the transistor is reverse biased.


Most MOSFETs are symmetric, so if Vds is negative you can just swap what you call source and drain. Source is always the node having the lowest voltage of the two nodes, and Vgs is reffered to that node.

However, if Vsb (source-bulk) is non-zero you should also take into account body effect.


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