US Patent 7348533 Unit pixel of CMOS image sensor

In a unit pixel of a CMOS image sensor which compensates for a dark current generated in a photo diode to enhance its driving range, the photo diode generates a charge in accordance with a received light amount. A drive transistor has a gate for receiving the charge in the photo diode to output as an electrical signal, and a drain to which a power voltage is applied. A saturation detector receives a gate voltage of the drive transistor and judges the drive transistor saturated if an output voltage is smaller than a preset reference voltage. A switch connects or disconnects between the power voltage and the gate of the drive transistor in response to the judgment of the saturation detector.