[en] We performed various pulse measurements on an atomic layer deposited (ALD) HfO₂-based resistive switching random access memory (RRAM) device and investigated its electronic synaptic characteristics. Unlike requirements for RRAM device application, to achieve the multi-state conductance changes required for the synaptic device, we employed additional sputtered TaO_x thin film formation on the ALD HfO₂ switching medium, which leads to engineering the concentration of oxygen vacancies and modulating the conductive filaments. With this TaO_x/HfO₂ bi-layered device, we attained gradual resistive switching, linear and symmetric conductance change, improved endurance and reproducibility characteristics compared to a single HfO₂ device. Finally, we emulated spike-timing-dependent plasticity based learning rule with pulses inspired by neural action potential, indicating its potential as an electronic synaptic device in a hardware neuromorphic system. (paper)