An optical apparatus including a semiconductor substrate; a first light absorption region supported by the semiconductor substrate, the first light absorption region configured to absorb photons and to generate photo-carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches configured to collect at least a portion of the photo-carriers based on the first control signal; one or more second switches controlled by a second control signal, the one or more second switches configured to collect at least a portion of the photo-carriers based on the second control signal; and a counter-doped region formed in a first portion of the first light absorption region, the counter-doped region including a first dopant and having a first net carrier concentration lower than a second net carrier concentration of a second portion of the first light absorption region.