This paper reports some aspects of major practical interest now that GaN HEMTs are in the onset of commercialization for high voltage applications (V_B >; 300 V). The device reproducibility of 900 V-class MIS-HEMTs with a thin in-situ grown Si₃N₄ gate insulator is investigated by means of wafer mapping and high-T stress. A remarkable result is that an exceptional yield of 99% across an entire 4-inch AlGaN/GaN-on-Si wafer when a reverse test of V_ds = 100 V was performed. In the same sense, it was found an impressive low dispersion of the threshold voltage, the saturation voltage and the on-resistance for 120 devices. Besides, under a DC reverse test, the typical gate and drain leakage currents are negligible (I <; 1 μA/mm) up to 500 V and temperature independent (at the low drain bias) up to 250°C.