| Summary: | The transport and synaptic characteristics of the two-terminal Au/Ti/ amorphous Indium-Gallium-Zinc-Oxide (a-IGZO)/thin SiO<sub>2</sub>/p<sup>+</sup>-Si memristors based on the modulation of the Schottky barrier (SB) between the resistive switching (RS) oxide layer and the metal electrodes are investigated by modulating the oxygen content in the a-IGZO film with the emphasis on the mechanism that determines the boundary of the abrupt/gradual RS. It is found that a bimodal distribution of the effective SB height (Φ<sub>B</sub>) results from further reducing the top electrode voltage (<i>V</i><sub>TE</sub>)-dependent Fermi-level (<i>E</i><sub>F</sub>) followed by the generation of ionized oxygen vacancies (V<sub>O</sub><sup>2+</sup>s). Based on the proposed model, the influences of the readout voltage, the oxygen content, the number of consecutive V<sub>TE</sub> sweeps on Φ<sub>B</sub>, and the memristor current are explained. In particular, the process of V<sub>O</sub><sup>2+</sup> generation followed by the Φ<sub>B</sub> lowering is gradual because increasing the <i>V</i><sub>TE</sub>-dependent <i>E</i><sub>F</sub> lowering followed by the V<sub>O</sub><sup>2+</sup> generation is self-limited by increasing the electron concentration-dependent <i>E</i><sub>F</sub> heightening. Furthermore, we propose three operation regimes: the readout, the potentiation in gradual RS, and the abrupt RS. Our results prove that the Au/Ti/a-IGZO/SiO<sub>2</sub>/p<sup>+</sup>-Si memristors are promising for the monolithic integration of neuromorphic computing systems because the boundary between the gradual and abrupt RS can be controlled by modulating the SiO<sub>2</sub> thickness and IGZO work function.
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