I do not think the use of the SG15 basis set has much to do with this issue, unless you have a particular explanation for that, so there is no need to modify the basis set.
I do not understand why you do H-passivation of H-atoms for the very central region part of the device only, not the entire device. It might be that your system has no band gap because there exist many dangling bonds giving rise to multiple defect levels inside the gap.
The electrode extensions appear to be too short. That might be a reason for the warning you got in the log file.
I would suggest first doing electronic structure calculations for a single layer to understand what the band structure of your system looks like without the external field applied, and how it depends on the passivation method. I guess all that also depends on the actual atomic structure of the system, i.e., whether it is in equilibrium or strained.
