Effects of channel thickness on electrical properties and stability of zinc tin oxide thin-film transistors

We investigated the effects of channel thickness (t_ch) on the electrical properties and the bias stress-induced instability of zinc tin oxide (ZTO, Zn : Sn = 4 : 1) thin-film transistors (TFTs). With increasing t _ch, the electrical properties of the TFTs showed a trend: threshold voltage (V_th) decreased and field-effect mobility (μ_FE) increased; on the other hand, subthreshold swing and hysteresis characteristics did not show any significant change. The positive bias stress (PBS) test and the following analysis by the stretched-exponential equation revealed that the PBS-induced threshold voltage shift (ΔV_th) decreased and the extracted characteristic trapping time (τ) increased with increasing t _ch. This thickness-dependence of PBS instability cannot be explained by charge trapping/injection model or defect creation model but can be well explained by less ambient effect at the backchannel due to less gate field through a thicker film.