Image sensor and forming method of image sensor pixel structure

The invention provides an image sensor and a forming method of an image sensor pixel structure, and is applied to the technical field of semiconductors. A part of a gate structure of a transmission transistor in a pixel structure is expanded into a photodiode region in a semiconductor substrate by forming a trench, so that the width of the photodiode can be extended in the transverse direction while the effective area of the gate structure of the transmission transistor is increased, the electron transmission communication area is increased, the photo-induced electron transmission rate is improved, the electron residue at the bottom of the diode is reduced, and the full-well capacity of the photodiode is finally improved. The manufacturing cost of the chip can be reduced under the condition that the area and lighting of the chip are not influenced. In addition, due to the fact that the electron transmission communication area in the pixel structure is increased, electron residues at the bottom of the diode ar