Semiconductor device having buried gate structure and method for fabricating the same

A semiconductor device, comprising:

• a substrate including an active region defined by an isolation layer;
• a buried gate structure provided in a trench formed in the substrate;
• and
• a first doped region and a second doped region formed in the active region and separated by the trench,
• wherein the buried gate structure comprises:
• a gate dielectric layer conformally covering the trench;
• and
• a gate electrode including a first portion partially filling the trench on the gate dielectric layer and a second portion formed on the first portion,
• wherein the second portion comprises a material included in the first portion and dopants including phosphorous (P), germanium (Ge), or a combination thereof,
• wherein the first portion does not laterally overlap with the first doped region and the second doped region, and all or a part of the second portion laterally overlaps with the first doped region and the second doped region, and
• wherein sidewalls of the first and the second portions are aligned with each other, and the first portion has a height greater than that of the second portion.