We have used scanning tunneling microscopy, atom-resolved tunneling spectroscopy, and photoemission to investigate the interaction of B with Si (111). Using decaborane as the source of B, we have followed the structural and electronic modifications of the surface as a function of the annealing temperature. In the stable B/Si (111)-√3 X √3 surface, B occupies a novel configuration where it substitutes for a Si atom in the 3rd atomic layer directly below a Si adatom. Because of a Si-to-B charge transfer, the top Si adatom layer has no occupied dangling-bond states and is insulating. As a result, the chemical properties of Si adatoms on the B/Si (111)-√3 X √3 surface are very different from those of the adatoms on the Si(111) -7 × 7 surface. We find evidence for doping effects on chemistry that involve short-range direct dopant-reactive site interactions. Finally, we report on the electrical characteristics of localized defect sites on the B-doped Si surface. We found that I-V curves over such sites may show regions of negative differential resistance and that this behavior is localized in areas of atomic dimensions (~1 nm). We propose a model according to which the development of negative resistance requires the existance of narrow peaks at appropriate energies in the density of states spectra of both sample and tip. © 1990, American Vacuum Society. All rights reserved.