A semiconductor structure includes a photodetector, which includes a substrate semiconductor layer having a doping of a first conductivity type, a second-conductivity-type photodiode layer that forms a p-n junction with the substrate semiconductor layer, a floating diffusion region that is laterally spaced from the second-conductivity-type photodiode layer, and a transfer gate electrode including a lower transfer gate electrode portion that is formed within the substrate semiconductor layer and located between the second-conductivity-type photodiode layer and the floating diffusion region. The transfer gate electrode may laterally surround the p-n junction, and may provide enhanced electron transmission efficiency from the p-n junction to the floating diffusion region. An array of photodetectors may be used to provide an image sensor.