Field-effect transistors based on the low-cost GaN-on-Si platform are promising candidates for highefficiency power switching at high frequencies. We have reported a normally-off GaN-on-Si switch with a blocking voltage of 1200V, and a very low dynamic on-resistance . For future improvement of the GaN-on-Si switching technology, it is important to understand the role of the non-insulating Si-substrate in device characteristics. In this paper, we discuss the static (DC) and dynamic (switching) characteristics of the GaN-on-Si device, focusing on the impact of bias conditions applied on the Si substrate. It was found that state-of-the-art dynamic on-resistance characteristics of the GaN-on-Si switch can be achieved by properly terminating the Si substrate potential.