Outstanding GaN-based HFETs (HFET: Heterojunction Field-Effect Transistors)  power devices are expected to replace all Si power devices in high power applications such as inverter systems due to their excellent performance. In order to exploit the full potential of such emerging GaN power devices, the gate driver that controls the device by a pulse width modulation (PWM) signal is becoming more important. The vital function of the gate driver is to provide an isolated gate signal against the reference source voltage that operates at high voltage. In addition to this function, their integration with GaN power HFETs is also desirable to achieve smaller system size, lower cost and user-friendliness. Although there are several signal isolation techniques for a gate driver such as to use a photo-coupler and wireless pulse transformer , these techniques have disadvantages such as large system size and difficulty in integration. Other bootstrap or charge pump techniques  in high voltage gate drivers (HVIC) have been developed to generate a reference voltage, but the driver can only be used in particular applications such as inverters. Meanwhile, we have focused our attention on the recent technology of wireless power transmission using an electromagnetic resonant coupler (EMRC)  with an open-ring resonator , which is very attractive due to its high efficiency in power transmission and its compactness at high frequency.