Effects of Programming-Pulse Frequency on Synaptic Characteristics of Amorphous IGZO Thin-Film Transistors With Defective Gate Oxides

We present a study on synaptic behaviors dependent on the programming-pulse frequency (PPF) of amorphous In-Ga-Zn-O thin-film transistors (a-IGZO TFTs). Specifically, weight-update and retention characteristics (e.g., dynamic ratio and retention time) with respect to different PPFs of fabricated a-IGZO TFTs are investigated, respectively. Here, for the memory functionality of a-IGZO TFTs, the defective oxide (i.e., SiOx) deposited at low temperature enables electron trapping and de-trapping. For the experiment based on the memory function, the positive programming-pulses with three different PPFs (e.g., 0.125 Hz, 0.25 Hz, and 0.5 Hz) are applied, while keeping the total programming energy constant. As a result, an appropriate dynamic ratio (e.g., 266) and longer retention time (e.g., 9×103sec) for the PPF = 0.25 Hz are observed. This is because of a relatively low energy per programming pulse for 0.5 Hz and long recovery at the read process for 0.125 Hz, respectively. Besides this experiment, since it is expected that different pulse heights can affect the synaptic characteristics, another experiment for each PPF is performed maintaining the programming pulse area. When the PPF is reduced from 0.25 Hz, it is found that both the dynamic ratio and retention time are increased. This is because a positive bias stress for applying a longer programming time can suppress the electrons de-trapping toward the channel as well as further moves to the deep trap states, implying the enhanced retention characteristics.