Semiconductor device and method for fabricating the same

1. A method for fabricating a semiconductor device, comprising:

forming an isolation region by etching a predetermined portion of a semiconductor substrate and filling the predetermined portion with an insulating material;

forming a gate by patterning a gate oxide layer and a gate electrode layer on the semiconductor substrate for each of a PMOS transistor and an NMOS transistor;

forming lightly doped source/drain regions at both sides of the gate by implanting impurity ions into the semiconductor substrate at a low density, wherein the implanting impurity ions for the PMOS transistor and the NMOS transistor have opposite conductivities;

forming heavily doped source/drain regions by implanting impurity ions into the semiconductor substrate adjacent to the lightly doped source/drain regions at a high density;

selectively forming a silicide layer on the gate and the source/drain regions; and

forming a nitride layer on an entire upper surface of the semiconductor substrate.

2. The method of claim 1 , further comprising:

implanting silicon ions (Si + ) in the semiconductor substrate after forming the nitride layer on the entire upper surface of the semiconductor substrate.

3. The method of claim 2 , wherein the silicon ions (Si + ) are implanted at an energy of approximately 20 to 130 KeV to a concentration of approximately 1×10 14 to 1×10 15 ions/cm 2 .

4. A semiconductor device, comprising:

an isolation region formed in a semiconductor substrate that electrically isolates an N-type MOS transistor area from a P-type MOS transistor area;

a gate formed on the semiconductor substrate corresponding to each of the N-type MOS transistor area and the P-type MOS transistor area;

lightly doped source/drain regions formed in the semiconductor substrate at both sides of the gate;

heavily doped source/drain regions formed in the semiconductor substrate and adjacent to the lightly doped source/drain regions; and

a nitride layer formed on an entire upper surface of the semiconductor substrate, wherein said nitride layer has silicon ions (Si + ) selectively implanted in the P-type MOS transistor area.

5. The semiconductor device of claim 4 , wherein the N-type MOS transistor area has no silicon ions (Si + ) implanted.

6. The semiconductor device of claim 4 , wherein said nitride layer of the N-type MOS transistor area has no silicon ions (Si + ) implanted.

7. The semiconductor device of claim 4 , further comprising:

a suicide layer formed on upper surfaces of the gate and the source/drain regions.

8. A semiconductor device, comprising:

an isolation region formed in a semiconductor substrate defining a plurality of active regions;

a plurality of gates formed on the substrate of the plurality of active regions;

a plurality of lightly doped source/drain regions formed in the substrate corresponding to both sides of each of the plurality of gates;

a plurality of heavily doped source/drain regions formed in the semiconductor substrate and adjacent to the lightly doped source/drain regions;

a silicide layer formed on an upper surface of each of the plurality of gates and the plurality of source/drain regions; and

a nitride layer formed on an entire upper surface of the substrate.

9. The semiconductor device of claim 8 , wherein one of the plurality of gates and one of the plurality of the lightly doped and heavily doped source/drain regions constitutes an N-type MOS transistor formed in an N-type MOS transistor area.

10. The semiconductor device of claim 8 , wherein one of the plurality of gates and one of the plurality of the lightly doped and heavily doped source/drain regions constitutes a P-type MOS transistor formed in a P-type MOS transistor area.

11. The semiconductor device of claim 10 , wherein the nitride layer is formed by implanting silicon ions (Si + ) in the P-type MOS transistor area.