Abstract GaN/AlGaN high electron mobility transistors (HEMTs) were fabricated on layer structures grown by metal organic chemical vapor deposition on hydride vapor phase epitaxy grown AlN epi-layers on 6H–SiC substrates. The presence of the AlN provides an insulating buffer for effective inter-device isolation, producing isolation currents in the 10−9 A range. These initial HEMTs exhibit saturated drain source current of >400 mA/mm with maximum transconductance of >70 mS/mm. The devices show lower degrees of current collapse relative to more conventional HEMTs fabricated on sapphire substrates, suggesting that the lower defect density is beneficial in reducing surface state trap concentration.