Gate bias instability of hydrogenated amorphous SiGe thin-film transistors

The gate bias dependence of the threshold voltage shift in a-Si (x) Ge1-x :H thin-film transistors (TFTs) was examined by a comparison with a-Si:H TFTs. A lower on-current and a small hole current at the off region were observed in the a-Si (x) Ge1-x :H TFTs, which were attributed to the larger number of defects and higher band gap density below the Fermi level. The threshold voltage shift was larger for the a-Si (x) Ge1-x :H TFTs than for the a-Si:H TFTs under both positive and negative bias stress due to defects that transitioned from the weak bond of the tail states, as confirmed by measuring the conductance activation energy of the a-Si (x) Ge1-x :H TFTs. In particular, the amount of the shift was smaller in the case of negative bias stress, which could be explained by the lower carrier density. The larger shift of the threshold voltage for the a-Si (x) Ge1-x :H TFT was attributed to the larger tail of the band gap states originating from the weak bonds.