In this study, AlGaN/GaN high-electron-mobility transistors with a 5-nm p-GaN cap layer were investigated to compare their performance under various activation conditions. Specifically, p-GaN cap layers were activated using rapid thermal annealing at 700◦C for 5, 10, and 15 min in an N2 environment before device fabrication. The gate leakage current reduced considerably when the p-GaN cap layer activation time was longer. The measured on/off current ratio was improved to 9 × 107 for a Schottky-gate device with 15-min annealing time. The breakdown voltage was increased using the activated p-GaN cap layer. In pulsed I-V measurements, the device with the p-GaN cap layer with a 15-min activation time exhibited less current dispersion.