Strange problem when working with small currents in ADE L

Hi,

I noticed a problem when simulating in ADE L with a 0.18um technology. For a simple current mirror, I found that results are not reliable for small currents (in the order of nA) if transistors’ parameters W and L are directly set. On the contrary, if the multiplier parameter is used, simulation results match with those expected. Moreover, for currents in the order of μA, simulation results are always as expected.

The schematic is this one:

Results of 10 DC operating point simulations are shown in these Tables. As you can guess by noticing the VDS,sat value, the level of inversion is kept more or less constant across simulations. For small current, only setting the multiplier parameter works. On the contrary, for higher currents, either using multiplier of directly setting the values of transistors work as expected. It is also worth noting that scaling the Lenght of the transistor instead of the Width WORKS both for small and high currents.

What could be happeining here?

Thanks in advanced!