A method of making a GaN device includes: forming a GaN substrate; forming a plurality of spaced-apart first metal contacts directly on the GaN substrate; forming a layer of insulating GaN on the exposed portions of the upper surface; forming a stressor layer on the contacts and the layer of insulating GaN; forming a handle substrate on the first surface of the stressor layer; spalling the GaN substrate that is located beneath the stressor layer to separate a layer of GaN and removing the handle substrate; bonding the stressor layer to a thermally conductive substrate; forming a plurality of vertical channels through the GaN to define a plurality of device structures; removing the exposed portions of the layer of insulating GaN to electrically isolate the device structures; forming an ohmic contact layer on the second surface; and forming second metal contacts on the ohmic contact layer.