GaN metal-oxide-semiconductor high electron mobility transistors (MOS)HEMTs) offer outstanding properties for next-generation power electronics devices. The high conductivity, high voltage blocking capability, high operation frequency, and device-level integration can be achieved on the same tech-nology platform. Moreover, the development of large-scale GaN-on-Si substrate reduces the cost of GaN lateral devices. That GaN power device has already been widely used in today's consumer electronics, and communications base stations with its superiority of power density, and energy efficiency. Despite the ideal side of GaN material for power transistor, the junction less structure of HEMT made it hard to achieve enhancement-mode (e-mode) operation. Existing solutions either rely on complicated processes or sacrifice device performance. A simple process, high performance, and reliable operation e-mode device are desired. This thesis proposes Li doped NiO as gate material combined with Tri-gate structure to achieve e-mode operation. This relied on a simple oxide deposition process, without using barrier recess or re-growth, and achieved a high-quality interface. These critical characteristics haven't been demonstrated on gate stack engineering for normally-off devices and reveal the outstanding stability of LiNiO as a junction gate. Moreover, to maximize the GaN power transistor performance boundary, a multi-channel junction gate device was developed, which uses multiple 2DEG channels to significantly reduce the device resistance while still operating at high breakdown voltage (VBR). The multi-channel junction gate de-vice presents state-of-the-art performance, stable operation, and simple process e-mode GaN transistor. Apart from achieving e-mode operation, the possibility of device-level integration was also explored. Reverse current conduction ability was achieved together with good on-state performance and voltage blocking capability. And the alternative method of regulating electric field to Tri-gate by grayscale lithography was developed. The results in this thesis reveal the great value of using oxide semiconductor LiNiO as junction gate, demonstrating high performance, e-mode, and reliable device, and unleashing the performance potential through multi-channel epitaxy.