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In this letter, we investigate the On/Off switching mechanism of AlGaN/GaN Fin-high-electron-mobility transistors (Fin-HEMTs) comprehensively through experiment and simulation. The "tri-gated fin channel" is characterized by a direct gate-metal/semiconductor contact, that is, a Schottky gate instead of a conventional metal-insulator-semiconductor gate stack. The minimum fin width of our Fin-HEMT is 100 nm with a threshold voltage (Vth) of −0.65 V, and a positive Vth shift with a channel width scaling is also obtained in the experiment. Through the 3-D simulation, it can be found that while the channel width is narrow enough, the carrier in the fin channel is dominated by the side-gate laterally instead of the vertical top-gate control. The band diagram also indicates that the conduction band in the fin channel is pulled up more rapidly than the planar HEMT with a negative gate bias. This result can be attributed to channel pinch-off through the depletion region which is created by the Schottky side-gate. Therefore, a narrow fin channel can lead to the "early pinch-off effect" compared with the planar HEMT. [ABSTRACT FROM AUTHOR]