GaN is very promising for power switching transistors taking advantages of the high breakdown strength with high saturation electron velocity. The lateral and compact device configuration enables high speed switching with reduced on-state resistance and parasitic capacitance. In this paper, state-of-the-art device technologies of GaN transistor and its monolithic integration for switching applications are reviewed. The topics include a novel normally-off transistor called Gate Injection Transistor (GIT) fabricated on a cost-effective Si substrate, and thermally stable isolation by Fe ion implantation. These are applied for the world first monolithic GaN inverter IC for motor drive with high efficiency. The presented technologies are indispensable for wide-spread use of GaN power switching transistors in the future.