In this paper, we present the first 3D Si MOS-capacitor-based radiation (gamma ray) sensor with high sensitivity and wide dose operation for occupational dosimetry applications. The capacitance changes almost linearly with the total radiation dose. By 3D structuring of Si surface with an anisotropic wet etching, surface area increases by 4 times in comparison with the 2D flat surface. Consequently, radiation sensitivity increases by more than 3 times, which is significantly better than previous reported sensitivity for MOS capacitor type radiation sensors.