Wide bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) are the promising materials for next-generation power devices. A normally-on GaN-based high-electron-mobility transistor (GaN HEMT) has been fabricated for power electronic converters. In this study, the current collapse phenomena, which are distinctive characteristics of GaN devices, are evaluated in detail under several voltages with two switching frequencies. Prototype circuits for a boost-type dc-dc converter and a single-phase full-bridge inverter, which use the newly fabricated GaN HEMT, are constructed and tested. The experimental results demonstrate that the newly fabricated normally-on GaN HEMT with the previously proposed gate drive circuit is suitable for use as a next-generation power device.