The I-V characteristics of graphene nanoribbon (GNR) p-n junctions have been investigated using atomistic quantum simulation. On the basis of results obtained for simple armchair GNR structures with large bandgap, it is suggested to improve significantly the device operation by inserting a small-bandgap section in the transition region between n and p zones. A giant peak-to-valley ratio (PVR) of negative differential conductance (higher than 103) can be achieved in such hetero-junctions. Additionally, the PVR is proved to be weakly sensitive to the transition length and not strongly degraded by the edge disorder, which is an important feature regarding applications.