A semiconductor device includes a dielectric structure, a first source/drain electrode, a second source/drain electrode, an oxide semiconductor layer, a gate dielectric layer, and a first gate electrode. The first source/drain electrode is disposed in the dielectric structure. The oxide semiconductor layer is disposed on the first source/drain electrode in a vertical direction. The second source/drain electrode disposed on the oxide semiconductor layer in the vertical direction. The gate dielectric layer is disposed on the dielectric structure and surrounds the oxide semiconductor layer in a horizontal direction. The gate dielectric layer includes a first portion and a second portion. The first portion is elongated in the horizontal direction. The second portion is disposed on the first portion and elongated in the vertical direction. The first gate electrode is disposed on the first portion of the gate dielectric layer.