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A normally- OFF AlGaN/GaN high-electron-mobility transistor (HEMT) with p-GaN gate was fabricated on free-standing GaN substrate. The self-terminated etching technology was achieved by combining with AlN stop layer and SF6-based etching gas. Compared with HEMT on Si substrate (Si-HEMT), the HEMT on GaN substrate (GaN-HEMT) demonstrated the higher current density, lower subthreshold swing (SS), lower drain leakage current, and lower static and dynamic ON-resistance that were ascribed to the lower dislocation and less defect on free-standing GaN substrate. Meanwhile, the excellent ${V}_{\mathrm{th}}$ stability up to 175 °C and uniformity was demonstrated. Also, higher breakdown voltage (BV) of 683 V was measured, which was higher than 507 V for Si-HEMT. Hence, a normally- OFF AlGaN/GaN HEMT on free-standing GaN substrate shows good potential in power application. [ABSTRACT FROM AUTHOR]