A new microscopic model allowing us to calculate the entropy of evaporation is introduced. The entropy of evaporation, has been calculated for the variety of solid substances, such as metals, elemental and compound semiconductors. The entropy of evaporation, calculated for the solids with various lattice symmetries is in good agreement with the existing thermodynamic data. An electronic formulation of the enthalpy of evaporation of semiconducting solids will be given. This formulation will be used in the analysis of solid-vapor equilibrium conditions of elemental and III-V compound semiconductors. The criterion for metal-rich and nonmetal rich growth conditions will be formulated in terms of partial pressures of the constituents of compound semiconductors. The criterion will be used to describe the conditions of growth gallium nitride (GaN) and aluminum nitride (AlN) from their vapor constituents. The metal/nonmetal rich criterion explains success of the growth of AlN single crystals from Al and N-2 vapor and disappointing results of the growth of GaN from the vapor constituents.