The semiconductor device includes a semiconductor substrate, an isolation feature, a photodiode and a transistor gate. The isolation feature is disposed in the semiconductor substrate. The photodiode is disposed in the semiconductor substrate and adjacent to the isolation feature. The photodiode includes a first pinned photodiode (PPD) with a first dopant type and a second PPD with a second dopant type. The second PPD is embedded in the first PPD, and is different from the first dopant type. The transistor gate is disposed over the photodiode and includes a first portion and a second portion. The first portion with the first dopant type is used for controlling the operation of the semiconductor device. The second portion with the second dopant type is adjacent to the first portion. The second portion covers the photodiode and extends toward the isolation feature.