In this study， we report a new design of GaN metal insulation semiconductor - high electron mobility transistor （MIS-HEMT） device with a 5 nm high-quality SiN_X dielectric layer deposited between gate and AlGaN barrier layer， to reduce the gate reverse leakage and improve power added efficiency （PAE）. Superior characteristics of the device are proved in DC， small signal and large signal tests， showing the improved device owing a high-quality interface， a wide-control-range gate， the capability to control current collapse and the ability to maintain high PAE when serving at frequency higher than 5 GHz. Serving at 5 GHz with V_DS = 10 V， the device showed an output power of 1.4 W/mm， with PAE of 74.4%； when V_DS rises to 30 V， output power increases to 5.9 W/mm with PAE remaining at 63.2%； a high PAE （50.4%） remained even when the test frequency increased 30 GHz while keeping the same output power. Additionally， the high-quality gate dielectric layer allows the device to withstand a wide gate voltage swing： the gate current remained 10^-4 A/mm even gain compressed to 6 dB. The results demonstrate the improvement of the SiN_x on MIS-HEMT device， which provides device-level guarantee for the power application of the system and the design of broadband circuits.