Thin film transistor and method of fabricating thin film transistor substrate

1. A method of fabricating a thin film transistor (TFT) comprising:

forming a polysilicon active layer on a substrate;

forming a gate insulating layer to cover the polysilicon active layer;

forming a first gate electrode on the gate insulating layer;

doping a first impurity into the polysilicon active layer to form a channel region on which the first gate electrode is superposed and a pair of first lightly doped drain (LDD) regions disposed at the both sides of the channel region;

forming a second gate electrode on the first gate electrode to completely cover the first gate electrode;

doping a second impurity into the polysilicon active layer to form a pair of second LDD regions having an impurity concentration higher than an impurity concentration of the first LDD regions, and source and drain regions opposite to each other interposing the channel region, the first LDD regions and the second LDD regions between them; and

forming source and drain electrodes connected to the source and drain regions, respectively.

2. The method of claim 1 , wherein the second gate electrode has a width larger than a width of the first gate electrode.

3. The method of claim 2 , wherein the source and drain regions have an impurity concentration higher than the impurity concentration of the second LDD regions.

4. The method of claim 3 , further forming a gate insulating pattern, the forming of the second gate electrode and the forming the gate insulating pattern comprising:

forming a gate metal layer on the substrate on which the channel region and the first LDD regions are formed;

forming a photoresist pattern on the gate metal layer;

over-etching the gate metal layer such that the gate metal layer has a width narrower than a width of the photoresist pattern to form the second gate electrode; and

etching the gate insulating layer using the photoresist pattern as a mask to form the gate insulating pattern having a width larger than a width of the second gate electrode.

5. The method of claim 3 , further comprising forming a gate insulating pattern, the forming of the second gate electrode and the gate insulating pattern comprising:

forming a gate metal layer on the substrate on which the channel region and the first LDD regions are formed;

forming a stepped photoresist pattern on the gate metal layer;

etching the gate metal layer and the gate insulating layer using the stepped photoresist pattern as a mask to form the second gate electrode and the gate insulating pattern;

ashing the stepped photoresist pattern to expose parts of the second gate electrode; and

etching the exposed second gate electrode using the ashed photoresist pattern as a mask.

6. A method of fabricating a thin film transistor (TFT) substrate comprising:

forming a polysilicon active layer on a substrate;

forming a gate insulating layer to cover the polysilicon active layer;

forming a first gate electrode on the gate insulating layer;

doping impurities into the polysilicon active layer to form a channel region and a pair of first LDD regions;

forming a second gate electrode completely covering the first gate electrode;

doping impurities into the polysilicon active layer to form a pair of second LDD regions having an impurity concentration higher than an impurity concentration of the first LDD regions, and source and drain regions having an impurity concentration higher than the impurity concentration of the second LDD regions;

forming an interlayer insulating film to cover the second gate electrode and exposing the source and drain regions;

forming source and drain electrodes connected to the exposed source and drain regions, respectively; and

forming a pixel electrode connected to the drain electrode,

wherein the first gate electrode superposes on the channel region.

7. The method of claim 6 , wherein after the forming of the second gate electrode and prior to the forming of the pair of second LDD regions and the source and drain regions, further comprising:

forming a gate insulating pattern and wherein the second gate electrode has a width larger than a width of the first gate electrode.

8. The method of claim 7 , wherein the forming of the second gate electrode and the gate insulating pattern comprises:

forming a gate metal layer on the substrate on which the channel region and the first LDD regions are formed;

forming a photoresist pattern on the gate metal layer;

over-etching the gate metal layer such that a width of the gate metal layer becomes narrower than a width of the photoresist pattern, to form the second gate electrode; and

etching the gate insulating layer using the photoresist pattern as a mask to form the gate insulating pattern having a width larger than a width of the second gate electrode.

9. The method of claim 7 , wherein the forming of the second gate electrode and the gate insulating pattern comprises:

forming a gate metal layer on the substrate on which the channel region and the first LDD regions are formed;

forming a stepped photoresist pattern on the gate metal layer;

etching the gate metal layer and the gate insulating layer using the stepped photoresist pattern as a mask to form the second gate electrode and the gate insulating pattern;

ashing the stepped photoresist pattern to expose parts of the second gate electrode; and

etching the exposed second gate electrode using the ashed photoresist pattern as a mask.

10. The method of claim 6 , further comprising forming a gate line on the substrate, forming a data line to cross the gate line on the substrate, and forming at least one of a gate driver for driving the gate line and a data driver for driving the data line, the at least one driver including a plurality of transistors, wherein the gate line and the data line are connected to the TFT.

11. The method of claim 10 , wherein each of the transistors having a polysilicon active layer comprising a channel region, source and drain regions opposite to each other having the channel region disposed therebetween, and at least two LDD regions formed between the source region and the channel region and between the drain region and the channel region, the LDD regions having impurity concentration different from each other.