<p>Germanium–tin (GeSn) alloy, a potential direct bandgap semiconductor compatible with complementary metal oxide semiconductor (CMOS) processes, is a promising platform for silicon-based infrared photodetector integrated systems. The emergence of GeSn-based optoelectronic devices avoids the high cost and silicon incompatibility of traditional III–V and II–VI infrared materials. Starting with a brief overview of the fundamental constraints in material epitaxy and defect engineering, this review focuses on the structural engineering of GeSn infrared photodetector devices and its influence on performance. We systematically review the evolution of device architectures and their working principles, from basic p-i-n junctions to advanced quantum wells, nanowire(NW)/dots, and ultimately to waveguide-integrated and flexible platforms. This review then delves into state-of-the-art performance-enhancement strategies, including multiple quantum wells (MQWs), resonant-cavity enhancement (RCE), and light-trapping schemes enabled by nanostructuring. In conclusion, we summarize the core challenges confronting GeSn photodetector and present a forward-looking perspective on their future applications in high-speed optical communication, mid-infrared(MIR) sensing, quantum detection, and flexible electronics.</p>