Surface states have hindered and degraded many semiconductor devices since the Bardeen era. Surface states originate from dangling bonds on the surface. This paper discusses a generic solution to surface states, i.e. valence-mending passivation. For the Si(100) surface, a single atomic layer of valence-mending sulfur, selenium or tellurium can terminate ~99% of the dangling bonds, while group VII fluorine or chlorine can terminate the remaining 1%. Valence-mending passivation of Si(100) has been demonstrated using CVD, MBE and solution passivation. The keys to valence-mending passivation include an atomically-clean Si(100) surface for passivation and precisely one monolayer of valence-mending atoms on the surface. The passivated surface exhibits unprecedented properties. Electronically the Schottky barrier height between various metals and valence-mended Si(100) now follows more closely the Mott-Schottky theory. With metals of extreme workfunctions, new records for low and high Schottky barriers are created on Si(100). The highest barrier so far is 1.14 eV, i.e. a larger-than-bandgap barrier, and the lowest barrier is below 0.08 eV and potentially negative. Chemically silicidation between metal and valence-mended Si(100) is suppressed up to 500˚C, and the thermally-stable record Schottky barriers enable theirapplications in nanoelectronic, optoelectronic and photovoltaic devices. Another application is transition metal dichalcogenides. Valence-mended Si(100) is an ideal starting surface for growth of dichalcogenides, as it provides only van der Waals bonding to the dichalcogenide.