Described examples include a semiconductor device with a substrate (22), a low defect layer (32) formed in a fixed position relative to the substrate (22), and a barrier layer (34) including III-N semiconductor material formed on the low-defect layer (32) and forming an electron gas in the low-defect layer (32). The device also has a source contact (52), a drain contact (54), and a gate contact (56) for receiving a potential, the potential for adjusting a conductive path in the electron gas and between the source contact and the drain contact. Lastly, the device has a one-sided PN junction (22/24) between the barrier layer (34) and the substrate (22).