Surface states are a classic obstacle in semiconductor technologies dating back to the John Bardeen era. We propose a generic approach, i.e., valence-mending passivation, to remove surface states. This paper reviews valence-mending passivation of the Si(1 0 0) surface, which is accomplished by depositing a monolayer of chalcogen atoms on Si(1 0 0). Methods for preparing an atomically-clean surface and depositing a self-limited monolayer of chalcogen atoms on Si(1 0 0) are developed in molecular beam epitaxy, solution passivation, and chemical vapor deposition. The passivated surface exhibits unprecedented electrical and chemical properties that are atypical of three-dimensional bulk semiconductors. The Schottky barrier heights for various metals now obey the Mott-Schottky theory on valence-mended Si(1 0 0). Metals of very-low and very-high workfunctions produce record-high and record-low Schottky barriers on the passivated surface. The record-high barrier demonstrated is 1.14 eV for an Al/sulfur-passivated p-type Si(1 0 0) junction, which exceeds the bandgap of Si. The record-low barrier is lower than 0.08 eV for an Al/sulfur-passivated n-type Si(1 0 0) junction and that barrier is likely negative at -0.02 eV. These record Schottky barriers show good thermal stability up to 500 degrees C upon annealing. Potential applications of valence-mending passivation include: (1) new approaches to Ohmic contacts for both heavily-and lightly-doped semiconductors, (2) a new diode that is an intermediate between a Schottky junction and a p-n junction, (3) suppressed surface and grain boundary recombination in optoelectronics and photovoltaics, and (4) the ideal substrate for van der Waals epitaxy of two-dimensional materials. The limitations of the current methods in characterizing extremely-low and negative Schottky barriers are outlined.