We report on fully vertical GaN-on-Silicon P-i-N diodes delivering above 1200 V soft breakdown voltage. Temperature dependence measurements indicate avalanche breakdown capability reflecting the high-quality processing and epitaxy growth. On-state characteristics of the fabricated vertical P-i-N diodes reveal on-resistances ranging from 0.48 m.cm² for small anode to 1.7 m.cm² for large anode diameters (i.e. 1 mm). The on-state resistance increase is attributed to the thermal dissipation issues. Nevertheless, the large devices exhibit high on-state current close to 12 A owing to an optimized process including a deep mesa etch as edge terminations and thick Cu layer for heat sink on the backside enabled by a polyimide passivation that strengthen the mechanical robustness of the membranes. To the best of our knowledge, this represents the first demonstration of fully vertical 1200 V GaN-based devices grown on silicon substrates with high current operation above 10 A, corresponding to a Baliga figure of merit of 3 GW/cm².