Negative drain current on N-Mosfet

File: Silicon.png (16KB, 489x402px) Image search: [Google]

16KB, 489x402px

i'm trying to simulate an N-Mosfet in Comsol multiphysics (for personal education reasons) following some tutorials i found available. When i do it with silicon substrate, the graph i get for Vg vs Id is 'silicon.png" and can clearly see the turn on voltage is 1.2V as expected.
I changed the material out of curiosity to Galium Arsenide and the plot i get is "GaAs.png' with a turn on voltage around 2.2 V BUT with negative current.

I do the sweep for a Vd of 0.01V and Vg 0 to 4V with a step of 0.2V. I gave negative Vg values to see if it changes(-4:0.2:4 V) but still remains at ~0A until the 2.2V point.

Does the negative current indicate that the n-mosfet is reversed bias for that material or is it something else?