A III-nitride trench device has a vertical conduction region with an interrupted conduction channel when the device is not on, providing an enhancement mode device. The trench structure may be used in a vertical conduction or horizontal conduction device. A gate dielectric provides improved performance for the device by being capable of withstanding higher electric field or manipulating the charge in the conduction channel. A passivation of the III-nitride material decouples the dielectric from the device to permit lower dielectric constant materials to be used in high power applications.