Fabrication and Characterization of Proton-Gated Field-Effect Transistors Based on β-phase MoO<sub>3</sub> Epitaxial Thin Films

We developed proton-gated field-effect transistors (FETs) utilizing β-phase molybdenum trioxide (MoO3) films grown by molecular beam epitaxy for neuromorphic-device applications. MoO3, whose conductivity is significantly modulated by redox reactions, was used as the channel layer of the device. Nafion, which exhibits proton conductivity, was used as the solid electrolyte. A porous silica spacer was adopted to suppress the leakage current through the Nafion membrane. The FETs demonstrated an ON/OFF ratio of approximately 103 and synaptic conductance modulation in response to gate pulses. Short-term memory responses facilitated 3-bit pattern recognition, indicating the potential for implementation in physical reservoir computing.