STUDY OF THE 5TH SHIFT OF THE THIN-FILM TRANSISTOR BY THE BIAS TEMPERATURE STRESS TEST

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 (V(th)) shifts to higher values after prolonged operating times. The reason for this V(th) 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/SiN(x) interface. Our TFT insulator has the rather complicated structure of an anodic oxide film, SiO(x), SiN(x) sequentially stacked over the gate electrode, which makes it difficult to separate the contribution of each layer. To confirm the V(th) shift mechanism and the contribution of each layer of insulator to the V(th) shift, we have prepared samples with a series of different insulators and have measured the blas dependence of their V(th) shifts. Our results show that the anodic oxide film makes no contribution to the V(th) shift, and it makes little difference to the V(th) shift whether the next insulator is SiN(x) or SiO(x). The latter fact may be explained in two ways. One is that both SiN(x) and SiO(x) make the same contribution to the V(th) shift. Alternatively, neither SiN(x) nor SiO(x) makes any contribution to the V(th) 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 V(th) shift of our a-Si:H TFT.