Different types of Mg/p-Si diodes were prepared by thermal evaporation of thin Mg layers on H-passivated Si(111) surfaces at low temperatures followed by various annealing procedures. After deposition at 190 K, the current-voltage characteristics of the diodes are typical for interfaces with lateral inhomogeneity and high ideality factors. The homogeneous barrier height is measured as 0.8 eV. This value is much larger than expected from the metal-induced gap states and electronegativity concept. The deviation is explained by interface dipoles due to a monatomic hydrogen layer between Mg and Si. Hydrogen is removed by annealing the diodes to room temperature. This procedure leads to the formation of an intermediate Mg silicide layer and reduces the homogeneous barrier height to 0.69 eV. The annealed diodes show excellent current-voltage properties. Applications of the diodes for chemical sensing such as chemicurrents and for internal photoemission are discussed. (c) 2007 American Vacuum Society.