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We present a study on weight-update characteristics dependent on channel layer properties (i. e. , geometrical and material properties) of low-power Hf–ZnO synaptic thin-film transistors (Syn-TFTs). Here, the channel layer properties can affect deep state capacitance (C ₃₄₄), which leads to a change in the average subthreshold slope (SS ₀ₕ₆). This varied SSavg can also contribute to the sensitivity during synaptic processes (i. e. , synaptic depression and potentiation), thus different weight-update characteristics (e. g. , dynamic ratio, drₖ). To prove these, the static and pulsed characteristics of the Syn-TFTs with different channel layer properties (i. e. , channel thickness and composition ratio of HfO2/ZnO) are monitored. From experimental results, the Syn-TFT with a thinner channel is found to have a larger drw compared to the Syn-TFT with a thicker channel. This is mainly because the channel thickness is proportional to C ₃₄₄. In terms of material properties, it is observed that the Syn-TFT with a higher composition ratio of HfO2/ZnO has a larger drw in comparison with the Syn-TFT based on a lower composition ratio of HfO2/ZnO. This can be attributed to the strong oxygen bonding energy of Hf ions, which can inhibit the increase of oxygen vacancies in the channel layer, reducing C ₃₄₄. In this vein, the Syn-TFT with both a thinner thickness of the channel layer and a higher composition ratio of HfO2/ZnO can achieve the higher drw (i. e. , 60. 9), resulting in the average recognition rate (RR) of 93. 4%.
Ko et al. (Tue,) studied this question.