Image sensor device and forming method thereof

Some embodiments are directed towards an image sensor device. A photodetector is disposed in a semiconductor substrate, and a transfer transistor is disposed over photodetector. The transfer transistor includes a transfer gate having a lateral portion extending over a frontside of the semiconductor substrate and a vertical portion extending to a first depth below the frontside of the semiconductor substrate. A gate dielectric separates the lateral portion and the vertical portion from the semiconductor substrate. A backside trench isolation structure extends from a backside of the semiconductor substrate to a second depth below the frontside of the semiconductor substrate. The backside trench isolation structure laterally surrounds the photodetector, and the second depth is less than the first depth such that a lowermost portion of the vertical portion of the transfer transistor has a vertical overlap with an uppermost portion of the backside trench isolation structure. The embodiments of the invention furthe