A pixel-array substrate includes a floating diffusion region and a first photodiode formed in a semiconductor substrate. A top surface of the semiconductor substrate defines a trench 1A and a trench 1B each (i) extending into the semiconductor substrate away from a planar region of the top surface between the trench 1A and the trench 1B and (ii) having a respective distal end, with respect to the floating diffusion region, located between the floating diffusion region and the first photodiode. In a horizontal plane parallel to the top surface and along an inter-trench direction between the trench 1A and the trench 1B, a first spatial separation between the trench 1A and the trench 1B increases with increasing distance from the floating diffusion region.