Abstract: Diamond has emerged as a quintessential representative of next-generation semiconductor materials, owing to its ultra-wide bandgap, exceptional thermal conductivity, high breakdown field strength, and outstanding carrier mobility. It has thus attracted extensive attention from fields such as power electronics, radio-frequency communications, and quantum information technologies. Intrinsic single-crystal diamond serves as the foundational substrate for diamond semiconductor development, requiring impurity concentrations at the parts per billion (ppb) level and extremely low dislocation densities. While the high-pressure high-temperature (HPHT) method yields material of higher purity and superior crystal quality, its utility is limited by small crystal dimensions. Consequently, HPHT-grown diamond is frequently employed as a substrate for chemical vapour deposition (CVD) homoepitaxy, enabling the preparation of large-area, high-quality single crystals.Regarding doping, the boron (B) atom, with a size difference of merely 6.5% compared to carbon (C), readily incorporates into the diamond lattice, facilitating the production of high-performance p-type diamond. Related devices, such as Schottky barrier diodes, have been successfully demonstrated. In contrast, n-type doping presents a fundamental challenge: potential dopants like phosphorus (P) and sulphur (S) possess atomic radii 35–57% larger than carbon, making their incorporation and activation within the lattice exceedingly difficult. The ultra-high pressure and high-temperature diffusion method, which modulates this atomic size disparity under extreme pressures (e.g.,~15GPa), emerges as a promising new pathway towards achieving shallow-level n-type doping.Concerning surface terminations, hydrogen termination induces a high-mobility two-dimensional hole gas (2DHG), whilst oxygen termination enhances interface stability and provides chemical passivation. However, their thermal stability windows (approximately 400 °C and 600 °C, respectively) remain inferior to those of substitutionally doped diamond, limiting their application in high-temperature and high-frequency devices. Therefore, breakthroughs in n-type doping, enhanced thermal stability of surface terminations, and the development of large-area, cost-effective fabrication processes are critical to advancing diamond semiconductor technology towards commercialisation in power electronics, quantum technologies, and high-performance sensing. This review aims to analyse and discuss these pivotal issues, exploring both the prospects and the persistent challenges facing diamond semiconductor development.