Transistor having increased effective channel width

A method of manufacturing a transistor of an image sensor, comprising:

• forming a recess to a first depth in a substrate material, the recess corresponding to a gate region and extending in a channel length direction and a channel width direction that is perpendicular to the channel length direction;
• forming a trench structure in the substrate material by enlarging the recess to using an isotropic process such that the trench structure extends to a second depth, wherein a width of the trench structure changes from the first depth in the substrate material into the second depth of the substrate material, wherein the trench structure has a polygonal cross section defined in a channel width plane with at least four sidewall portions of the substrate material;
• forming a photodiode in the substrate material adjacent to the trench structure;
• forming an isolation layer on the substrate material;
• forming a gate portion of the isolation layer on the substrate material such that the gate portion of the isolation layer extends into the trench structure;
• and
• forming a gate on the isolation layer such that the gate extends into the trench structure.