An image sensor can include a gate insulation layer, a gate electrode, a photodiode, and a floating diffusion region. The gate insulation layer can be formed on and/or over a semiconductor substrate for a transfer transistor. The gate insulation layer includes a first gate insulation layer having a central opening and a second gate insulation layer formed on and/or over an uppermost surface of the first gate insulation layer including the opening. The gate electrode can be formed on and/or over the gate insulation layer. The photodiode can be formed in the semiconductor substrate at one side of the gate electrode so as to generate an optical charge. The floating diffusion region can be formed in the semiconductor at the other side of the gate electrode opposite to the photodiode. The floating diffusion region can be electrically connected to the photodiode through a channel so as to store the optical charge generated from the photodiode.