High performance AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistor was fabricated using HfO2 as the surface passivation and gate insulator. The gate and drain leakage currents are drastically reduced to tens of nanoamperes before breakdown. Without field plates, for 10 μm of gate-drain spacing, the off-state breakdown voltage is 1035 V with a specific on resistance of 0.9 mΩ cm2. In addition, there is no current slump observed from the pulse measurements. This is the best performance reported on GaN-based power-switching devices on sapphire up to now, which efficiently combines excellent device forward, reverse, and switching characteristics.