1. Field of the Invention
The invention relates to a flat panel display technology, and in particular to an improved thin film transistor (TFT) device having different electrical characteristics in driving circuit and pixel regions and a method for fabricating the same.
2. Description of the Related Art
The demand for active-matrix flat panel displays, such as active matrix organic light emitting device (AMOLED) displays, has increased rapidly in recent years. AMOLEDs typically employ thin film transistors (TFTs) as pixel and driving circuit switching elements which are classified as amorphous silicon (a-Si) TFTs and polysilicon TFTs according to the materials used as an active layer. Compared with a-Si TFTs, polysilicon TFTs have the advantages of high carrier mobility, high driving-circuit integration and low leakage current, and are often applied to high-speed operation applications. Thus, low temperature polysilicon (LTPS) is a novel application for FPD technology. LTPS allows for an easier IC manufacturing process, which integrates driving circuits on a glass substrate having pixels thereon, reducing the manufacturing cost.
In the LTPSTFT fabrication, the TFTs in the driving circuit region and the pixel region are fabricated at the same time and by the same process. Therefore, the TFTs in the pixel and driving circuit regions have the same electrical characteristics. In AMOLED, however, the electrical characteristics of the TFTs in the driving circuit region are different from that in the pixel region. For example, it is desirable to design driving TFTs with high carrier mobility and low sub-threshold swing for proving fast response. Additionally, it is desirable to design pixel TFTs with high sub-threshold swing to increase gray scale inversion of the AMOLED, thereby providing high contrast ratio. However, it is difficult to fabricate TFTs with high sub-threshold swing for a pixel region and low sub-threshold swing and high carrier mobility for a driving circuit region because they are fabricated at the same time and by the same process. That is, in conventional LTPSTFT fabrication, the electrical characteristics of TFTs in the driving circuit region are degraded when the gray scale inversion for the pixel region is increased. Conversely, the gray scale inversion for the pixel region is reduced when the electrical characteristics of TFTs in the driving circuit region are improved.
Thus, there exists a need in the art for development of an improved thin film transistor device which has different TFT electrical characteristics in the driving circuit and pixel regions, thereby providing pixel TFTs with high sub-threshold swing without degrading the electrical characteristics of drive TFTs.