Effects of Channel Layer Properties on Weight-Update Characteristics of Low-Power Hf-Doped ZnO Thin-Film Transistors

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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}_{\text {deep}}\text {)}$</tex-math> </inline-formula>, which leads to a change in the average subthreshold slope (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{SS}_{\text {avg}}\text {)}$</tex-math> </inline-formula>. This varied SS<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">avg</sub> can also contribute to the sensitivity during synaptic processes (i.e., synaptic depression and potentiation), thus different weight-update characteristics (e.g., dynamic ratio, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{dr}_{w}\text {)}$</tex-math> </inline-formula>. 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 HfO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>/ZnO) are monitored. From experimental results, the Syn-TFT with a thinner channel is found to have a larger dr<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</sub> compared to the Syn-TFT with a thicker channel. This is mainly because the channel thickness is proportional to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}_{\text {deep}}$</tex-math> </inline-formula>. In terms of material properties, it is observed that the Syn-TFT with a higher composition ratio of HfO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>/ZnO has a larger dr<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</sub> in comparison with the Syn-TFT based on a lower composition ratio of HfO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>/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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}_{\text {deep}}$</tex-math> </inline-formula>. In this vein, the Syn-TFT with both a thinner thickness of the channel layer and a higher composition ratio of HfO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>/ZnO can achieve the higher dr<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</sub> (i.e., 60.9), resulting in the average recognition rate (RR) of 93.4%.