First of all, thanks for your help Max!
Sorry, I should have sketched the substrate tap. The p-substrate is connected also to ground, so I am theoretically applying the voltage between well and substrate. At this point, I should mention that I am not 100% sure that the p-substrate connection to ground is well done in my layout, but I am more or less confident that it does is correct.
Besides, the source terminal is connected to/drain/gate/gnd in my drawing!
I can't figure why I am seeing a constant I/V rate if the nwell-psub forms a reverse-biased diode!
Thanks again.
David.