A complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) includes a semiconductor substrate including a photodiode therein as a light sensing unit. A floating diffusion region of a first conductivity type is provided in the semiconductor substrate, and is configured to receive charges generated in the photodiode. A power supply voltage region of the first conductivity type is also provided in the semiconductor substrate. A reset transistor including a reset gate electrode on a surface of the substrate between the floating diffusion region and a power supply voltage region is configured to discharge charges stored in the floating diffusion region in response to a reset control signal. The reset transistor includes a channel region in the substrate extending between the floating diffusion region and the power supply voltage region such that the floating diffusion region and the power supply voltage regions define source/drain regions for the reset transistor. An impurity region is provided in a first portion of the channel region adjacent to the floating diffusion region. The impurity region has a doping such that the first portion of the channel region adjacent to the floating diffusion region has a different built-in potential than a second portion of the channel region adjacent to the power supply voltage region.