Negative voltage to the drain of NMOS

Question

In NMOS, when,

            Vgs = Vth
            Vds < (Vgs - Vth = 0)
            so, Vds is -ve,

so initially I thought in nmos, if Vds is -ve, Id direction reverses and electron flow from drain to source, in the n channel, so reverse current increases.

I simulated in LTSpice and saw that reverse current is like -617Amps, But why it's that high? (Here I think some conceptually wrong occurred, what's that?)

Answer 1

For the 4-terminal NMOS that you used in your simulation, there is a PN junction between the transistor body (P type) and both the source and drain (each is N type).

So, when you bring the voltage on the drain below ground, with the body still connected at ground, you will forward bias the PN junction from the body to the drain. Like any other forward-biased PN junction, large currents will flow.

Answer 2

In the reverse-to-normal connection of the drain and source, there is a forward conducting diode. That probably accounts for the massive current. It's called a "bulk diode" and is a parasitic component inside the MOSFET that limits its application when reverse voltages are present.

MOSFET symbol showing the diode: -

Picture from here.