The buffer breakdown of a group III-N HEMT on a p-type Si substrate is significantly increased by forming an n-well in the p-type Si substrate to lie directly below the metal drain region of the group III-N HEMT. The n-well forms a p-n junction which becomes reverse biased during breakdown, thereby increasing the buffer breakdown by the reverse-biased breakdown voltage of the p-n junction and allowing the substrate to be grounded. The buffer layer of a group III-N HEMT can also be implanted with n-type and p-type dopants which are aligned with the p-n junction to minimize any leakage currents at the junction between the substrate and the buffer layer.