ltspicemosfet
I am testing the I V curve of the mosfet in LTSpice
I get strange results when the standard 'NMOS' componant is reverse biased, (when the source is more positive than the drain) I get tera-amps flowing into the drain and mili-amps out of the source, no current into the gate.
Is this because the model for the MOSFET is incomplete and only works for the positive I V quadrant, or could there some kind of bug in the software?
The only difference between an enhancement mode NMOS and a depletion mode NMOS is the value of Vt for which Id = 0, the formulas are the same.
If you understand how an enhancement NMOS works you could just imagine it having a voltage source in series with the gate and the total will behave as a depletion NMOS !
This is defined as a subcircuit:
.SUBCKT PSMN2R0_30PL DRAIN GATE SOURCE
LTSpice needs this to have somewhat special treatment, so you will need to do the following:
CTRL+Right click on the device and you will get this window:
Now edit the Prefix and Value lines: The prefix for a subckt is 'X'. The model name is precisely as defined in the lib file.
Now click OK. You will need to add a spice directive on your schematic:
.include PSMN2R0_30PL.lib This assumes it is in the same directory as the simulation circuit.
LTSpice should now be happy with the part.
Here is what you should see on the schematic:
You can, of course, add it to the LTSpice model tree, but I find it easier to use this method.
Best Answer
Most likely you observe the behaviour of the body diode that gets forward biased.