The higher mobility is needed for thin film transistor (TFT) mainly used to be applied in the larger size flat-panel displays (FPDs). The amorphous metal oxide TFT has mobility higher than 10 cm2/V¡Es and can substitute the poor mobility (<1 cm2/V¡Es) of traditional amorphous silicon TFT, which shows a great potential for the next generation. Due to the superior characteristics in amorphous metal oxide TFT, therefore, the amorphous metal oxide TFT has been studied extensively. Usually, the source/drain with island type device has a large overlapped/contact area that we cannot determine the exact electron path. That the sample of inverted stagger £\-IGZO TFTs with via type device has smaller contact area and can be estimated the electron path. In this thesis, the devices with different M1 overlaps etching stop layer (ESL) via distance, M2 £\-IGZO contact size and the fringe field effect are investigated. Although the characteristics of £\-IGZO TFTs have great performance, the electrical stability under illumination and long term bias stress are still a important issue to study before implement them into display. Thus, the devices with different structures that we mentioned previously are investigated the electrical reliability which are the negative bias stress of gate voltage, hot carrier stress effect and negative bias of illumination. The electron path of via type is extracted by contact resistance which is greater than the distance between S/D via. Experiment results show that the increased offset between M1 and ESL via generates the resistance-liked effect in electrical characteristics. The hot carrier stress effect is independent of M2 £\-IGZO contact size in short channel length devices and there are close depletion lengths in drain side. The negative bias stress of illumination is proceeded in the fringe field effect devices, which results a negative shift of threshold voltage due to the hole trapping.