A Bias-Dependent Weight Update Characteristics of Low Power Synaptic Pass-Transistors with a Hf-Doped ZnO Channel Layer

A study on a bias voltage-dependent weight update characteristics in a sub-threshold region of a low power synaptic pass-transistor (SPT) is presented with a Hf-doped zinc oxide active layer. The SPT is a synaptic thin-film transistor (TFT) in series with a load TFT which is used as a resistive load (R_L) to be scaled by a bias voltage (V_B). Here, when the V_B of the load TFT is modulated, the R_L can be changed. With the changed R_L, it is expected that the weight update characteristics (i.e., dynamic ratio) and electrical characteristics (i.e., power consumption) of the SPT are varied, respectively, suggesting a trade-off relation between the dynamic ratio and power consumption. To check these, the pulsed characteristics of the fabricated SPT is monitored for different V_B, respectively. From experimental results, as increasing V_B, it is found that the decreased R_L leads to the increase of the power consumption while enhancing the dynamic ratio because a full depression (FD) can be relatively easy. On the other hand, when the V_B is reduced, the R_L is increased resulting in the decrease of both the power dissipation and the dynamic ratio due to a difficulty of FD.