Gate structures and semiconductor devices including the same

A gate structure, comprising:

• a first conductive pattern including a first metal compound and being doped with a first metal or silicon;
• a second conductive pattern on the first conductive pattern, the second conductive pattern including a second metal;
• and
• a third conductive pattern on the second conductive pattern, the third conductive pattern being doped with a fourth metal and including a third metal or a second metal compound,
• wherein a work function of the first metal is smaller than a work function of the first metal compound.

A semiconductor device, comprising:

• a substrate;
• an active pattern on the substrate;
• an isolation pattern covering a sidewall of the active pattern;
• a gate structure extending through upper portions of the active pattern and the isolation pattern in a first direction substantially parallel to an upper surface of the substrate;
• a bit line structure on a central portion of the active pattern, the bit line structure extending in a second direction substantially parallel to the upper surface of the substrate;
• a contact plug structure contacting each opposite end portions of the active pattern;
• and
• a capacitor structure on the contact plug structure;
• wherein:
• the gate structure includes:
• a first conductive pattern including a first metal compound, the first conductive pattern being doped with a first metal or silicon,
• a second conductive pattern on the first conductive pattern, the second conductive pattern including a second metal,
• a third conductive pattern on the second conductive pattern,
• a gate mask on the third conductive pattern, and
• a gate insulation pattern covering a lower surface and a sidewall of the first conductive pattern, a sidewall of the second conductive pattern, and a sidewall of the third conductive pattern, and
• a work function of the first metal is smaller than a work function of the first metal compound.