In high-electron-mobility transistor (HEMT) terahertz detectors, an excessively wide gate can generate oblique modes in the channel, resulting in weakened resonant detection signals and a broadened resonance peak. To address this issue, a side-gate HEMT (EdgeFET) structure was proposed. A resonant detection model for the side-gate device was established based on the hydrodynamic equations of the two-dimensional electron gas (2DEG) in conventional HEMT. A side-gate HEMT detector was fabricated, and terahertz resonant detection experiments were conducted at 77 K. The experimental results indicated that EdgeFET demonstrated distinct resonant responses at 77 K, with the resonant responsivity reaching 3.7 times the maximum non-resonant responsivity. The experimental data were fitted using the theoretical model to validate its accuracy. These results strongly confirm the effectiveness of EdgeFET in enhancing the resonant performance of the detector, providing a new technological approach for the development of next-generation high-performance terahertz detectors.