A unit pixel includes a sensing transistor, a photo diode, and a reset drain region. The sensing transistor includes a reference active region, an output active region, and a gate. The gate is between the reference active region and the output active region to electrically connect the reference active region to the output active region based on a gate voltage. The reference active region and output active region are within a semiconductor substrate. The photo diode is under the gate within the semiconductor substrate. The reset drain region is within the semiconductor substrate and is electrically connected to the photo diode by the gate based on the gate voltage.