Silicon, diamond, and their compound silicon carbide play pivotal roles in electronic science and technology. Trivalent impurities such as B and Al are the typical choices for $p$-type doping in these semiconductors. Intriguingly, B dominates in Si and diamond, while Al is preferred in SiC for industrial applications, despite their chemical similarity. Using first-principles calculations, we perform a comparative analysis of B- and Al-related defects across the three hosts. We show that in the elemental hosts, B is favored due to its lower $2p$ orbital energy than Al $3p$ orbital energy. In SiC, the ionicity leads to strong localization of the valence-band maximum on the C sublattice, rendering large perturbation and high acceptor level if substituting on the anion C site. At the cation Si site, Al exhibits a small size mismatch, a shallower transition level, and a lower formation energy, making it the more efficient dopant. Interstitial defects are either energetically unfavorable (in Si and diamond) or contribute similar compensation effects for B and Al (in SiC), and thus do not alter the observed trends. These results not only provide a unified atomistic explanation for acceptor doping in Si, diamond, and SiC, but also offer a physically transparent picture that links dopant efficiency to band-edge character, site preference, and lattice relaxation.

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• Point defects
• Doped semiconductors
• First-principles calculations