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Does anyone know how to model a depletion mode N channel MOSFET in spice?

I have looked online however the majority of resources have shown how to model a depletion type or enhancement type. I specifically want to model a FET that is fully on when there is a 0 Vgs voltage.

Resources i've looked at

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  • \$\begingroup\$ Can you explain the distinction you're making between "mode" and "type" in this context? \$\endgroup\$ – Dave Tweed Jan 21 '15 at 19:06
  • \$\begingroup\$ @DaveTweed In this context I am looking to model a fet where when Vgs is 0, current will flow freely between the drain and source. When Vgs > Vth the channel will close an only leakage will flow. Does that explain? \$\endgroup\$ – secretformula Jan 21 '15 at 19:14
  • \$\begingroup\$ Can you give a link to the material you found about depletion "type" MOSFETs? \$\endgroup\$ – The Photon Jan 21 '15 at 19:17
  • \$\begingroup\$ @ThePhoton the second one mentions depletion mode devices be I believe it's just referring to p channel fets. Also please tell me if there's a better nomenclature for these parts / behaviors \$\endgroup\$ – secretformula Jan 21 '15 at 19:24
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The solution is given in the LTSpice help file you linked:

VTO is positive (negative) for enhancement mode and negative (positive) for depletion mode N-channel (P-channel) devices.

For an NMOS FET, if VTO is positive, you have an enhancement mode device. If VTO is negative, then you have a depletion mode device.

For a depletion mode NMOS FET, with 0 Vgs the channel will conduct. Vgs must be pulled below 0 V in order to cut off the channel. In SPICE this behavior is represented by a negative threshold voltage, controlled by the VTO parameter.

From your comments, though, you want

When Vgs > Vth the channel will close an only leakage will flow.

To get this behavior, you need a p-channel depletion mode MOSFET. An n-channel FET doesn't work this way.

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