This work presents recent advances in the development of gallium arsenide (GaAs) solid-state microdosimeters based on semiconductor PIN diodes, designed for applications in proton therapy. The device architecture consists of an array of interconnected PIN (p–intrinsic–n) diodes geometrically distributed across a GaAs substrate, effectively emulating a cellular matrix to enable spatial resolution at the scale of biological cells. The structures are grown via molecular beam epitaxy (MBE) to precisely define the doping profiles, followed by microfabrication processes to create sensitive volumes comparable in size to individual cells, allowing for high-resolution microdosimetric measurements. Although our research explores both silicon- and GaAs-based devices, this work focuses on the GaAs development line, emphasizing recent progress in device fabrication and early-stage evaluation. Electrical characterizations have been conducted, demonstrating promising performance in terms of signal response and diode behavior. Final characterization using proton and heavy-ion beams will assess their full potential for medical applications and enhance the experimental infrastructure of the future Argentine Proton Therapy Center, supporting both research and clinical implementation.