A phototransistor used for an image sensor is provided. The phototransistor can reduce a dark current that occurs in the phototransistor and improve sensitivity at low luminance without crosstalk with a neighboring pixel or an image lag by including a buried collector. In the phototransistor including the buried collector, since the collector is not directly connected to outside, the phototransistor has a low dark current and a high photosensitive characteristic at low luminance. Since each image sensor is isolated, crosstalk between pixels or an image lag does not occur.