A study on the synaptic characteristics dependent on programming-pulse properties, such as the programming pulse width (PPW), of amorphous In-Ga-Zn-O thin-film transistors (a-IGZO TFTs) is presented. To validate this, the weight-update characteristics with respect to PPWs (e.g., 1.5 s, 3 s, and 6 s) are monitored. Here, the memory functionality of a-IGZO TFTs can be achieved with defects (e.g., trap states) in the gate insulator deposited at a low temperature process, leading to the electron trapping or de-trapping phenomena. Based on this function, when the programming pulses with same duty cycle and total time are applied to the gate terminal, the dynamic ratio for PPW = 3 s is found to be 18.5, which is larger compared to other PPWs. This is because the energy per programming-pulse for PPW = 1.5 s is relatively low whereas the recovery during the read process for PPW = 6 s is faster, degrading the weight-update characteristics. In order to fairly compare this recovery effect for each PPW, another pulsed experiment with different duty cycles is conducted maintaining the same number of programming pulses. As a result of the experiment, when the PPW is increased while keeping the same recovery condition, it is found to be a larger dynamic ratio due to a higher energy per programming pulse. With results of this device level, the analog AI accelerator simulation is also performed monitoring the recognition accuracy for three PPW cases.
Ko et al. (Thu,) studied this question.