Near-UV/Violet-Edge ZTO:Ga/Ag Thin-Film Transistor Photosensor With Integrated Readout: A Three-Dimensional Semiconductor–Wave Optics–Circuit Co-Simulation

This work presents, to the best of our knowledge, one of the first fully integrated three-dimensional device-circuit co-simulations of an indium-free gallium-doped zinc tin oxide (ZTO:Ga) thin-film transistor photosensor with silver electrodes and an integrated readout, implemented in COMSOL Multiphysics by coupling the Semiconductor, Wave Optics, and Electrical Circuit modules. The 420 × 300 × 135 nm device uses a 60 nm SiO2 gate dielectric, a 20 nm Ag gate, and a two-layer graded ZTO:Ga channel (lower 20 nm, N D = 5 × 10 16 cm −3 ; upper 35 nm, N D = 2 × 10 17 cm −3 ). Operating at the near-UV/violet edge, the device achieves a peak responsivity of 3.1 A W −1 at 380 nm through trap-mediated photogating gain ( G PG about 11). Transient simulations give exponential time constants of 3.8 ms (rise) and 5.5 ms (fall). The transimpedance amplifier (1 MΩ, C f = 0.1 pF) has a 1.59 MHz electrical bandwidth, while the complete-system bandwidth is limited to about 29 Hz by the slower device response. Using a full noise budget, the detectivity is about 6.9 × 10 8 Jones and the noise-equivalent power about 4.2 × 10 −14 W Hz 0.5 . The indium-free channel reduces supply-chain and toxicity concerns, and the co-simulation provides quantitative, physics-based performance projections before fabrication.