[Kwant] GNR_FET

Hi all, Kwant is a package to study transport in nano structures. I try to use it in device simulation viz. GNR FET etc. Gate voltage can be given by furnishing the potential profile in the channel under the gate. But how the drain voltage would be given? is it by giving energy distribution in the channel between the source and drain? Further, is it possible to specify geometry of the device structure like (i) source and drain extensions etc.? Further, how to proceed to study dependence of mobility on doping? Regards. K.K.Ghosh