Ohmic Contact Resistance in Wide-Bandgap and Ultrawide-Bandgap Power Semiconductors: From Fundamental Physics to...

Ohmic contact resistance is a persistent and increasingly dominant bottleneck limiting the practical performance of wide-bandgap (WBG) and ultrawide-bandgap (UWBG) power semiconductor devices. This review provides a comprehensive and comparative treatment of specific contact resistivity (ρc) phenomena across five material systems-4H-SiC, GaN, β-Ga₂O₃, AlN/AlGaN, and diamond-spanning fundamental contact physics, characterization methodology, material-specific state of the art, device context, and advanced engineering strategies. A semi-empirical scaling analysis establishes that the minimum achievable ρc increases by approximately one order of magnitude per 0.8-1.0 eV increase in bandgap, arising from the interplay of Fermi-level pinning, increasing carrier effective mass, and decreasing achievable near-surface doping concentration. The best demonstrated ρc values range from ~3 × 10^-8 Ω·cm² for GaN epitaxially regrown contacts to ~8 × 10^-5 Ω·cm² for direct AlN metallization. The transition from alloyed to regrown contacts in GaN-delivering two orders of magnitude improvement-is identified as the paradigm model for UWBG contact development, with β-Ga₂O₃ most immediately positioned to follow this trajectory. Key challenges include the absence of p-type doping in β-Ga₂O₃, near-complete Fermi-level pinning in AlN, and the unsolved shallow-donor problem in diamond. Recommendations for standardized ρc measurement protocols and priority research directions are presented.