Method of fabricating polysilicon, thin film transistor, method of fabricating the thin film transistor, and organic light emitting diode display device including the thin film transistor

1. A thin film transistor comprising:

a substrate;

a buffer layer disposed on the substrate;

a first semiconductor layer comprising a lowermost surface disposed on the substrate, the first semiconductor layer further comprising an uppermost surface opposite the lowermost surface, a first side surface, and a second side surface;

a second semiconductor layer disposed directly on the uppermost surface, the first side surface, and the second side surface of the first semiconductor layer, the second semiconductor layer having a larger surface area than the first semiconductor layer;

a gate electrode disposed on the substrate;

a gate insulating layer disposed on the substrate, between the gate electrode and the first and second semiconductor layers; and

source and drain electrodes connected to the second semiconductor layer, the source and drain electrodes being insulated from the gate electrode,

wherein a channel region of the second semiconductor layer is disposed on the entire uppermost surface of the first semiconductor layer.

2. The thin film transistor of claim 1 , wherein the first semiconductor layer and the second semiconductor layer are polysilicon layers formed using a metal catalyst.

3. The thin film transistor of claim 1 , wherein the first semiconductor layer and the second semiconductor layer are crystallized by super grain silicon (SGS) crystallization.

4. The thin film transistor of claim 1 , wherein the crystal grains of the second semiconductor layer are larger than the crystal grains of the first semiconductor layer.

5. The thin film transistor of claim 1 , wherein:

the buffer layer is disposed directly on the substrate;

the first semiconductor layer is disposed directly on the buffer layer;

the second semiconductor layer is disposed directly on the first semiconductor layer and the buffer layer;

the gate insulating layer is disposed directly on the second semiconductor layer and the buffer layer; and

a gate electrode is disposed directly on the gate insulating layer, facing the first and second semiconductor layers.

6. The thin film transistor of claim 1 , wherein:

the buffer layer is disposed directly on the substrate;

the gate electrode is disposed directly on the buffer layer;

the gate insulating layer is disposed directly on the gate electrode and the buffer layer;

the first semiconductor layer is disposed directly on the gate insulating layer, facing the gate electrode;

the second semiconductor layer is disposed directly on the first semiconductor layer and the gate insulating layer; and

the source and drain electrodes are disposed directly on the second semiconductor layer.

7. A method of fabricating a thin film transistor, comprising:

forming a buffer layer on a substrate;

forming a lowermost surface of a first semiconductor layer on the buffer layer, the first semiconductor layer further comprising an uppermost surface opposite the lowermost surface, a first side surface, and a second side surface;

forming a second semiconductor layer directly on the uppermost surface, the first side surface, and the second side surface of the first semiconductor layer, the second semiconductor layer having a larger surface area than the first semiconductor layer;

forming a gate insulating layer on the second semiconductor layer;

forming a gate electrode on the gate insulating layer; and

forming source and drain electrodes on the gate insulating layer, the source and drain electrodes being connected to the second semiconductor layer,

wherein the second semiconductor layer is a polysilicon layer that is crystallized by a metal catalyst contained in the first semiconductor layer,

wherein a channel region of the second semiconductor layer is disposed on the entire uppermost surface of the first semiconductor layer.

8. The method of claim 7 , wherein the forming of the first and second semiconductor layers comprises:

forming a first amorphous silicon layer on the buffer layer;

forming a metal catalyst layer on the first amorphous silicon layer;

annealing the first amorphous silicon layer to form a first metal catalyst crystallization region;

removing the metal catalyst layer;

patterning the first metal catalyst crystallization region to form the first semiconductor layer;

forming a second amorphous silicon layer on the first semiconductor layer;

annealing the second amorphous silicon layer to form a second metal catalyst crystallization region; and

patterning the second metal catalyst crystallization region to form the second semiconductor layer.

9. The method of claim 8 , wherein crystallizing the first amorphous silicon layer is performed after a diffusion layer is formed between the first amorphous silicon layer and the metal catalyst layer.

10. The method of claim 8 , wherein the annealings are performed at a temperature of from about 350° C. to 500° C.

11. The method of claim 8 , wherein the metal catalyst layer is formed of one selected from the group consisting of Ni, Pd, Ag, Au, Al, Sn, Sb, Cu, Tr, and Cd.

12. A method of forming a thin film transistor, comprising:

forming a buffer layer on a substrate;

forming a gate electrode on the buffer layer;

forming a gate insulating layer on the gate electrode and the buffer layer;

forming a lowermost surface of a first semiconductor layer on the gate insulating layer, the first semiconductor layer facing the gate electrode and comprising an uppermost surface opposite the lowermost surface, a first side surface, and a second side surface;

forming a second semiconductor layer directly on the uppermost surface, the first side surface, and the second side surface of the first semiconductor layer, the second semiconductor layer having a larger surface area than the first semiconductor layer; and

forming source and drain electrodes on opposing sides of the second semiconductor layer, the source and drain electrodes being connected to the second semiconductor layer,

wherein the second semiconductor layer is a polysilicon layer crystallized by a metal catalyst contained in the first semiconductor layer,

wherein a channel region of the second semiconductor layer is disposed on the entire uppermost surface of the first semiconductor layer.

13. The method of claim 12 , wherein the forming of the first and second semiconductor layers comprises:

forming a first amorphous silicon layer on the gate insulating layer;

forming a metal catalyst layer on the first amorphous silicon layer;

annealing the first amorphous silicon layer to form a first metal catalyst crystallization region;

removing the metal catalyst layer;

patterning the first metal catalyst crystallization region to form the first semiconductor layer;

forming a second amorphous silicon layer on the first semiconductor layer;

annealing the second amorphous silicon layer to form a second metal catalyst crystallization region; and

patterning the second metal catalyst crystallization region to form the second semiconductor layer.

14. The method of claim 13 , further comprising forming a diffusion layer between the first amorphous silicon layer and the metal catalyst layer, prior to the annealing of the first amorphous silicon layer.

15. The method of claim 13 , wherein the annealings are performed at a temperature of from about 350° C. to 500° C.

16. The method of claim 14 , wherein the metal catalyst layer is formed of any one selected from the group consisting of Ni, Pd, Ag, Au, Al, Sn, Sb, Cu, Tr, and Cd.

17. An organic light emitting diode (OLED) display device comprising:

the thin film transistor of claim 1 ;

an insulating layer disposed on the source and drain electrodes;

a first electrode disposed on the insulating layer and electrically connected to one of the source and drain electrodes;

an organic film layer disposed on the first electrode; and

a second electrode disposed on the organic film layer.

18. The device of claim 17 , wherein the first semiconductor layer and the second semiconductor layer are polysilicon layers formed using a metal catalyst.

19. The device of claim 17 , wherein the first semiconductor layer and the second semiconductor layer are crystallized by super grain silicon (SGS) crystallization.

20. The device of claim 17 , wherein the crystal grains of the second semiconductor layer are larger than the crystal grains of the first semiconductor layer.

21. An organic light emitting diode (OLED) display device comprising:

the thin film transistor of claim 5 ;

an insulating layer disposed on the source and drain electrodes;

a first electrode disposed on the insulating layer and electrically connected to one of the source and drain electrodes;

an organic film layer disposed on the first electrode; and

a second electrode disposed on the organic film layer.

22. An organic light emitting diode (OLED) display device comprising:

the thin film transistor of claim 6 ;

an insulating layer disposed on the source and drain electrodes;

a first electrode disposed on the insulating layer and electrically connected to one of the source and drain electrodes;

an organic film layer disposed on the first electrode; and

a second electrode disposed on the organic film layer.