Ultrawide-bandgap (UWBG) semiconductors, with bandgaps significantly wider than GaN's 3.4 eV, represent a promising area of research in semiconductor materials, physics, devices, and applications. These materials offer potential advantages in high-power and RF electronics, deep-UV optoelectronics, quantum information, and extreme-environment applications. Recent advancements in materials like high Al-content AlGaN, diamond, and Ga₂O₃ have made realizing these advantages more feasible. This article surveys the materials, physics, device, and application research opportunities and challenges for advancing UWBG semiconductors. UWBG materials include AlGaN/AlN, diamond, Ga₂O₃, and others. These materials have bandgaps significantly wider than GaN, with AlN reaching up to 6.0 eV. The performance of these materials scales nonlinearly with bandgap, leading to potential for far superior performance than conventional wide-bandgap (WBG) materials. For example, the Baliga figure of merit (BFOM) for power devices scales with the sixth power of the bandgap, meaning that moving from GaN to AlN could increase BFOM by a factor of 34. AlGaN/AlN, diamond, and Ga₂O₃ are the leading UWBG materials. AlGaN/AlN has good physical properties, including a wide range of direct bandgaps, high breakdown fields, and high electron mobility. However, challenges include doping and the need for high-quality single-crystal substrates. Diamond has excellent properties for high-power and high-frequency electronics, radiation detectors, and thermionic emitters. It has a high breakdown field, high electron and hole mobilities, and high thermal conductivity. However, challenges include the availability of large-area, low-defect-density substrates and efficient doping. Ga₂O₃ has a wide bandgap and good n-type conduction, but it is less mature and faces challenges in p-type doping and device fabrication. The article discusses the research opportunities and challenges for advancing UWBG semiconductors, including the need for better substrates, doping techniques, and device fabrication. It also highlights the potential of UWBG materials in various applications, such as power electronics, optoelectronics, and quantum information. The article concludes that UWBG semiconductors are at a stage of development similar to WBG materials in the 1980s, with many research challenges but also significant opportunities for performance improvements.Ultrawide-bandgap (UWBG) semiconductors, with bandgaps significantly wider than GaN's 3.4 eV, represent a promising area of research in semiconductor materials, physics, devices, and applications. These materials offer potential advantages in high-power and RF electronics, deep-UV optoelectronics, quantum information, and extreme-environment applications. Recent advancements in materials like high Al-content AlGaN, diamond, and Ga₂O₃ have made realizing these advantages more feasible. This article surveys the materials, physics, device, and application research opportunities and challenges for advancing UWBG semiconductors. UWBG materials include AlGaN/AlN, diamond, Ga₂O₃, and others. These materials have bandgaps significantly wider than GaN, with AlN reaching up to 6.0 eV. The performance of these materials scales nonlinearly with bandgap, leading to potential for far superior performance than conventional wide-bandgap (WBG) materials. For example, the Baliga figure of merit (BFOM) for power devices scales with the sixth power of the bandgap, meaning that moving from GaN to AlN could increase BFOM by a factor of 34. AlGaN/AlN, diamond, and Ga₂O₃ are the leading UWBG materials. AlGaN/AlN has good physical properties, including a wide range of direct bandgaps, high breakdown fields, and high electron mobility. However, challenges include doping and the need for high-quality single-crystal substrates. Diamond has excellent properties for high-power and high-frequency electronics, radiation detectors, and thermionic emitters. It has a high breakdown field, high electron and hole mobilities, and high thermal conductivity. However, challenges include the availability of large-area, low-defect-density substrates and efficient doping. Ga₂O₃ has a wide bandgap and good n-type conduction, but it is less mature and faces challenges in p-type doping and device fabrication. The article discusses the research opportunities and challenges for advancing UWBG semiconductors, including the need for better substrates, doping techniques, and device fabrication. It also highlights the potential of UWBG materials in various applications, such as power electronics, optoelectronics, and quantum information. The article concludes that UWBG semiconductors are at a stage of development similar to WBG materials in the 1980s, with many research challenges but also significant opportunities for performance improvements.