The present invention introduces a separation type unit pixel of an image sensor having a three-dimensional (3D) structure, which is capable of maximizing transmission efficient of a charge generated through a photodiode to a floating diffusion area. The separation type unit pixel may include a first wafer on which a photodiode and a transmission transistor are formed and a second wafer on which a reset transistor and a source follower transistor are formed. In particular, the photodiode has a positive region to which an N_ground voltage is applied, the N_ground voltage having a lower voltage level than a ground voltage used in the second wafer.