Achieving an ultra-broad spectral response in a self-bias detector is a formidable challenge that persists in optoelectronics that necessitates innovative solutionsWe propose a unique ultra-broadband photodetecting device, utilizing a multilayer structure comprising Molybdenum Di-sulfide (MoS2), Antimony Tri-selenide (Sb2Se3), and Gallium Nitride (GaN), which exhibits the unique capability of detecting photons without applied bias.The fabricated device demonstrates exceptional sensitivity to a wide range of illumination wavelengths, spanning from UV-C (UV-C) to IR-B (IR-B).The design detector displays the highest photo-responsivity of 665 mAW-1 in photovoltaic mode and 3.89 x 105 mAW-1 in photoconductive mode.The designed detector also exhibits a minimal dark current of 90 nA and an extremely weak signal detection capability of ∼12 femto watt-hertz-1/2 at 6 V bias.Addnl., the thermal stability of the MoS2-Sb2Se3-GaN (Mo-Sb-Ga) multi-layer-based self-bias detector was explored.Under the self-bias conditions, the photodetector exhibits a stable behavior up to 250°C with a peak responsivity of 635 mAW-1.The thermal durability of the self-bias ultra-broadband photodetector indicates excellent potential for developing futuristic optoelectronic devices.Further, the performance of the developed detector was examined using Technol. Computer-Aided Design (TCAD) simulations, providing valuable insights into the device behavior and the transport of photo-generated carriers, enhancing our understanding of the device operation and enabling performance optimization for diverse applications.