Gallium‐Content‐Dependent Optical Synaptic Plasticity in Amorphous Ga‐Zn‐Sn‐O Thin‐Film Transistors

Optoelectronic synaptic transistors integrate optical sensing with history‐dependent conductance modulation for neuromorphic information processing. Here, ultraviolet‐responsive synaptic transistors were fabricated using indium‐free amorphous Ga‐Zn‐Sn‐O (a‐GZTO) films deposited from targets with nominal Ga contents of 0.5, 1, and 2 wt.%. The devices exhibited graded composition‐dependent electrical, photoresponsive, and synaptic characteristics, with 1 GZTO generally showing intermediate behavior. O 1s peak fitting indicated that the defect‐related component decreased with increasing Ga content, consistent with the observed photoresponse and retention trends. Under 365 nm stimulation, the devices demonstrated excitatory postsynaptic current, paired‐pulse facilitation, and spike‐number‐, duration‐, and rate‐dependent plasticity. Peak‐normalized transients showed greater retention at lower Ga contents. Electrical and single‐pulse optical characteristics were evaluated across five devices per composition to assess device‐to‐device variation. For 10 UV pulses of the same width and intensity, increasing the repetition frequency enhanced the relative EPSC gain, consistent with high‐pass‐filter‐like temporal processing at the device level. These findings establish nominal Ga content as a materials‐design parameter for tuning UV response, retention, and relaxation in planar a‐GZTO transistors.