We report the demonstration of p-NiO layers as junction termination extensions (JTEs) for realizing high-performance GaN power devices. The p-NiO was deposited by RF sputtering, a flexible process which enables to achieve high hole densities. In this work, we investigated the material and transport properties of p-NiO on GaN, and demonstrated its application as JTEs for GaN-on-Si Schottky barrier diodes (SBDs). p-NiO JTEs resulted in a similar ON-state behavior compared to a control SBD without any termination, while providing 1.9x-higher breakdown voltage of 443 V, the highest reported for SBDs on GaN-on-Si substrates.