In this paper， we demonstrated SiN_x/AlN/GaN metal-insulator-semiconductor high electron mobility transistors （MIS-HEMTs） with low noise and high linearity， by in-situ growth of SiN_x gate dielectrics on ultra-thin barrier AlN/GaN heterostructure. Deep-level transient spectroscopy （DLTS） shows a traps-level depth of 0.236 eV， a capture cross-section of 3.06×10^-19 cm^-2， and an extracted interface state density of 10¹⁰-10¹² cm^-2eV^-1， which confirms that the grown SiN_x can reduce the interface state. The devices exhibit excellent DC， small signal， and power performance， with a maximum saturation output current （I_dmax） of 2.2 A/mm at the gate voltage （V_gs） of 2 V and the gate length of 0.15 μm， a maximum current cutoff frequency （f_T） of 65 GHz， a maximum power cutoff frequency （f_MAX） of 123 GHz， a minimum noise figure （NF_min） of the device of 1.07 dB and the gain of 9.93 dB at 40 GHz. The two-tone measurements at the V_ds of 6V， yield a third-order intermodulation output power （OIP3） of 32.6 dBm， and OIP3/P_dc of 11.2 dB. Benefited from the high-quality SiN_x/AlN interface， the MIS-HEMTs exhibited excellent low noise and high linearity， revealing its potential in applications of millimeter waves.