GaN/AlGaN device technologies are reviewed aiming at the applications to power switching systems. SL (Super Lattice) capping and QA (Quaternary Alloy) over-layer techniques have been developed to reduce the on-resistance of GaN/ AlGaN HFET. Further, we achieved GaN on Silicon epitaxial growth technology with almost the same mobility keeping the same 2DEG density, which will make the cost comparable to conventional Si one. The experimentally obtained R on A of the FET is 1.9 mΩcm 2 , which is 14 times lower than that of Si ones. Additionally, a novel approach to realize enhancementmode operation of GaN/AlGaN FET is proposed using minority carrier injection by the gate.