Semiconductor device and method for forming the same

1. A semiconductor device comprising:

a trench in a semiconductor substrate;

a junction region disposed on two sides of the trench;

a first gate electrode buried in the trench in such a manner that the first gate electrode has a step difference at a top surface thereof and has a first workfunction; and

a second gate electrode disposed over the first gate electrode, the second gate electrode including a polycide material and having a second workfunction,

wherein the second gate electrode overlaps with the junction region, and

wherein the second gate electrode surrounds an upper portion of the first gate electrode.

2. The semiconductor device according to claim 1 , wherein the second workfunction is substantially the same as a workfunction of un-doped polysilicon.

3. The semiconductor device according to claim 1 , wherein the second gate electrode includes a silicide of doped polysilicon.

4. The semiconductor device according to claim 3 , wherein the doped polysilicon is doped with N + ions.

5. The semiconductor device according to claim 1 , wherein the first electrode is formed to have a symmetrical step structure such that both sides contiguous to the junction region are lower in height than a center part between the both sides.

6. The semiconductor device according to claim 1 , wherein the first gate electrode includes a metal material.

7. The semiconductor device according to claim 1 , further comprising:

a gate insulation film disposed over sidewalls of the trench; and

a barrier metal film disposed over the gate insulation film.

8. The semiconductor device according to claim 1 , wherein the first workfunction is higher than the second workfunction.