SPICE simulations with LTSPICE showed no current flows thru drain & source of an enhancement n-channel MOSFET with using its SPICE model - yet hooking up 5v 2A power supply across an actual device (IRLB3034PBF) yields full current flow with nothing hooked up to the gate, another words no voltage into the gate, it was entirely not attached. Is SPICE incorrect about current flow?


A MOSFET's gate is floating and well-insulated thanks to the oxide layer between the gate and channel. As a result, when it is completely disconnected, Vgs may very well be anything depending on handling and other effects. You could have very well deposited enough charge on the gate for the transistor to turn on, merely while handling it. It is almost never correct for the gate of your MOSFETs to be truly floating unless you are doing something highly specialized, such as producing flash memory. You should find that, when the gate is driven in a sensible manner, then SPICE and experimentation will roughly agree.

SPICE just sees the small capacitance and leaves the gate potential at its initial conditions unless influenced to do otherwise.

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  • \$\begingroup\$ Would grounding the gate via the earth ground power supply lug diminish Ids current flow? \$\endgroup\$ – user33915 Aug 18 '18 at 22:53
  • \$\begingroup\$ @user9762541 It depends on how the MOSFET is connected. Is it a low-side switch? High-side switch? Some other topology? The parameter that matters is the voltage between gate and source (Vgs), and you haven't specified what the FET's source pin is actually tied to. For an enhancement-type nFET, positive Vgs tends to cause the FET to conduct, while Vgs = 0 causes Ids to go to nearly zero when other conditions (temperature, Vds) are within the FET's absolute maximum ratings. \$\endgroup\$ – nanofarad Aug 18 '18 at 22:57
  • \$\begingroup\$ I verified, the ground lug effectively discharges the gate and the current diminishes, if touch the gate with my fingers or something similar the current goes to maximum then diminishes agian if I discharge it with the earth ground reference. \$\endgroup\$ – user33915 Aug 18 '18 at 23:03
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    \$\begingroup\$ @user9762541 That's good to know but it doesn't really make your inquiry very clear. That's completely expected if your ground lug shares potential with the FET's source. \$\endgroup\$ – nanofarad Aug 18 '18 at 23:04
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    \$\begingroup\$ I usually use floating outputs and the ground lug didnt help, I realized the current diminished only after the source and gate had the same ground reference which was not even earth ground reference. \$\endgroup\$ – user33915 Aug 19 '18 at 0:14

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