III-N semiconductor heterostructures on III-N epitaxial islands laterally overgrown from a mesa of a silicon substrate. An IC may include a III-N semiconductor device disposed on the III-N epitaxial island overhanging the silicon mesa and may further include a silicon-based MOSFET monolithically integrated with the III-N device. Lateral epitaxial overgrowth from silicon mesas may provide III-N semiconductor regions of good crystal quality upon which transistors or other active semiconductor devices may be fabricated. Overhanging surfaces of III-N islands may provide multiple device layers on surfaces of differing polarity. Spacing between separate III-N islands may provide mechanical compliance to an IC including III-N semiconductor devices. Undercut of the silicon mesa may be utilized for transfer of III-N epitaxial islands to alternative substrates.