Abstract
There has been a tendency to improve the characteristics of switches in AMLCD (active matrix liquid-crystal display) technology. Particularly, the large photo-induce leakage current is an issue that need to be imminently solved. In this thesis, the amorphous silicon thin film transistors (a-Si:H TFTs) with low photo-induced leakage current will be proposed and studied. A novel a-Si:H TFT structure has been demonstrated to suppress the leakage current under back-light illumination. The proposed TFT has an extra layer of heavily phosphorous-doped a-Si:H film capping on the side wall of a-Si:H island in the BCE light-shield structure. Thus, by excluding the effect of Schottky contacts between source/drain electrodes and undoped a-Si:H at the side edge of a-Si island, the leakage path can be effectively eliminated. Also, under the gate-side illumination, the proposed TFT structure shows a photo-induced leakage current as low as two orders of magnitudes, compared to the conventional a-Si:H TFT device. The superior characteristics of the proposed structure can potentially provide better visual quality for AMLCD products. Moreover, the novel a-Si TFT device exhibits higher effective carrier mobility than that of conventional one. For that reason, the high performance provides the potential of the proposed a-Si:H TFT to apply for AMLCD and AMOLED technology. In addition, an alternative method has been proposed to suppress photo leakage current of a-Si TFT operated at off-state. We introduce a deep level trap center, copper, into Si film to act as recombination centers. After dipping conventional BCE TFTs into CuSO4 solution, copper will diffuse in back channel. Experimental results have shown the photo leakage current of devices is lower than that of a-Si TFT without the introduction of Cu. Although the mobility is decreased slightly, the degradation of mobility is still tolerable. The mechanism is mainly based on the photo-field effect that more recombination centers induce less reduction of band bending under illumination.