A semiconductor device and a manufacturing method thereof are disclosed. The semiconductor device includes a silicon substrate, a spacer, a doped region, and a deep trench isolation (DTI). The silicon substrate has a deep trench. The spacer is formed on an upper portion of the sidewall of the deep trench. The doped region is formed on a lower portion of the sidewall of the deep trench. The deep trench isolation is formed in the deep trench.