A vertical-gate transfer transistor of an active pixel sensor (APS) is provided. The transistor includes a semiconductor substrate, a vertical trench extending into the semiconductor substrate, a dielectric lining the vertical trench, and a vertical gate filling the lined vertical trench. The dielectric includes a dielectric constant exceeding 3.9 (i.e., the dielectric constant of silicon dioxide). A method of manufacturing the vertical-gate transfer transistor, an APS including the vertical-gate transfer transistor, a method of manufacturing the APS, and an image sensor including a plurality of the APSs are also provided.