Abstract
An active pixel sensor includes a substrate, a photo sensing region, a peripheral circuit region and an isolation region. The photo sensing region and the peripheral circuit region are formed on the substrate. The isolation region is formed between the photo sensing region and the peripheral circuit region for isolating the photo sensing region and the peripheral circuit region. The photo sensing region induces photo current according to the received light. The peripheral circuit region includes a first transistor having a source connected to a bit line, a second transistor having a gate connected to the photo sensing region, a source connected to the drain of the first transistor and a drain connected to a voltage source, and a third transistor having a source connected to the photo sensing region and a drain connected to the voltage source. The first transistor is used to select whether to output data stored in the photo sensing region or not. The third transistor is used to reset the photo sensing region.