There is a growing need for radiation sensors, particularly in the fields of national security, nuclear medicine imaging, and monitoring of environmental radioactivity. These applications require sensors that are small, portable, inexpensive, large-area compatible, and consume less power. However, current radiation detection systems can be highly expensive, bulky, require high operating voltages and high temperature processing, and require materials that are in limited supply and are highly toxic. This drives the need for emerging semiconductor materials and device structures to help achieve these challenging goals. Emerging semiconductors in the past decade include transition metal dichalcogenides (TMDs) and perovskite structured semiconductors due to their unique electronic and optical properties. In this dissertation, TMDs and perovskites are demonstrated in photo-detecting and radiation detecting devices. Large-area deposition of the MoSe2 TMD material by pulsed laser deposition is demonstrated with physical and electrical characterization. A proof-of-concept MoSe2 based heterojunction diode is fabricated and characterized using I-V, C-V, and optoelectronic analysis for photo-sensing applications. Perovskite based heterojunction diodes are designed, fabricated, electrically characterized, and tested for alpha, gamma, and neutron detection. The perovskite device performance is optimized by varying the materials coupled with the perovskite, reducing the perovskite thickness to optimize radiation collection, and introducing a guard ring to limit the device leakage current, demonstrating the first reported perovskite based single-crystal neutron detector. This dissertation demonstrates the impact emerging semiconductors, such as TMDs and perovskites, have in the field of radiation detecting devices.