Transistor having dual work function buried gate electrode and method for fabricating the same

1. A transistor comprising:

a source region and a drain region separately formed in a substrate;

a trench defined in the substrate between the source region and the drain region; and

a gate electrode formed in the trench,

wherein the gate electrode includes:

a titanium nitride electrode filling a bottom portion of the trench;

a metal electrode formed in the trench and over the titanium nitride electrode; and

a doped polysilicon liner electrode disposed between the titanium nitride electrode and the metal electrode,

wherein the doped polysilicon liner electrode extends to between a first sidewall of the metal electrode and the source region,

wherein the doped polysilicon liner electrode further extends to between a second sidewall of the metal electrode and the drain region, and

wherein the doped polysilicon liner electrode surrounds the first sidewall, the second sidewall, and a bottom portion of the metal electrode.

2. The transistor according to claim 1 , further comprising:

a barrier layer disposed between the doped polysilicon liner electrode and the metal electrode.

3. The transistor according to claim 2 , wherein the metal electrode has resistance lower than the titanium nitride electrode, the barrier layer, and the doped polysilicon liner electrode.

4. The transistor according to claim 1 , wherein the titanium nitride electrode has a first work function, and

wherein the doped polysilicon liner electrode has a second work function lower than the first work function.

5. The transistor according to claim 1 , wherein the titanium nitride electrode has a first work function higher than a mid-gap work function of silicon, and

wherein the doped polysilicon liner electrode has a second work function lower than the mid-gap work function of silicon.

6. The transistor according to claim 1 , wherein the metal electrode include a metal-containing material, and

wherein the doped polysilicon liner electrode has a work function lower than the titanium nitride electrode.

7. The transistor according to claim 1 ,

wherein the doped polysilicon liner electrode is doped with an N-type impurity, and

wherein the doped polysilicon liner electrode has a work function lower than the titanium nitride electrode.

8. The transistor according to claim 1 ,

wherein the gate electrode is positioned at a level lower than a top surface of the substrate, and

wherein the transistor further includes:

a capping layer over the gate electrode; and

a gate dielectric layer between the gate electrode and a surface of the trench.

9. The transistor according to claim 1 , further comprising:

an isolation layer formed in the substrate to define an active region,

wherein the source region and the drain region are formed in the active region and separated from each other, and

wherein the trench is defined in the active region between the source region and the drain region, and extends into the isolation layer.

10. The transistor according to claim 9 , further comprising:

a recess region in the isolation layer under the trench; and

a fin region formed in the active region under the trench, wherein sidewalls of the fin region is exposed by the recess region,

wherein the gate electrode covers the fin region, and

wherein the titanium nitride electrode covers a top and sidewalls of the fin region, while filling the recess region.

11. The transistor of claim 1 ,

wherein the bottom portion of the trench is filled with only a single layer of the titanium nitride electrode.