A read voltage dependent synaptic characteristics of sub-threshold thin film transistors with a hf doped ZnO active layer

We present a quantitative analysis of synaptic characteristics dependent on a read voltage (V_read) of a sub-threshold operating Hf-ZnO synaptic thin-film transistor (Syn-TFT). As an higher case with V_read = 2.3 V near the threshold voltage (V_T), the output current is depressed with applying positive programming-pulses, which is due to a positive V_T shift associated with the electron trapping into the gate insulator. For a lower case with V_read = 0.6 V around the flat-band voltage (V_FB), negative programming-pulses are applied to get de-trapping electrons from the gate insulator, resulting in a facilitation of the output current associated with a negative V_T shift. During the weight-update process, it is observed that the programming-speed for a lower V_read is faster compared to a higher V_read due to the steeper sub-threshold slope. After the programming processes, the retention characteristics is monitored with applying constant V_read, showing that the effective retention time for a higher V_read is much longer than that for a lower V_read. This is because a higher positive bias stress can lead to a positive V_T shift, suppressing the recovery of trapped electrons. Here, it is found that the bias stress can be intentionally used to improve the retention time.