Study of the Vth shift of the thin-film transistor by the bias temperature stress test

Abstract

Amorphous silicon thin-film transistors (a-Si:H TFTs) are now widely used as the switching device in the active-matrix addressing of liquid crystal displays. One concern is the potential instability problems associated with the threshold voltage (Vth) shifts to higher values after prolonged operating times. The reason for this Vth shift has been widely discussed, and two models accounting for it have been suggested. One model explains the shifts by the trapping of electrons in the insulator, the other model by the creation of the metastable states at the a-Si:H/SiNx interface. Our TFT insulator has the rather complicated structure of an anodic oxide film SiOx, SiNx sequentially stacked over the gate electrode, which makes it difficult to separate the contribution of each layer. To confirm the Vth shift mechanism and the contribution of each layer of insulator to the Vth shift, we have prepared samples with a series of different insulators and have measured the bias dependence of their Vth shifts. Our results show that the anodic oxide film makes no contribution to the Vth shift, and it makes little difference to the Vth shift whether the next insulator is SiNx or SiOx. The latter fact may be explained in two ways. One is that both SiNx and SiOx make the same contribution to the Vth shift. Alternatively, neither SiNx nor SiOx makes any contribution to the Vth shift, but the a-Si:H/gate insulator interface has some contribution. From these experiments, it cannot be determined which of the proposed mechanisms is consistent with the behavior of the Vth shift of our a-Si:H TFT.