An image sensor pixel array includes a photoelectric conversion unit that has a second region in a substrate and vertically below a gate electrode of a transistor. A first region under a top surface of the substrate and above the second region supports a channel of the transistor. A color filter transmits a light via a light guide, the gate electrode and the first region to generate carriers collected by the second region. The gate electrode may be made thinner by a wet etch. An etchant for thinning the gate electrode may be introduced through an opening in an insulating film on the substrate. The light guide may be formed in the opening after the thinning. An anti-reflection stack may be formed at a bottom of the opening prior to forming the light guide.