A one-dimensional mathematical model for the deposition of silicon nitride thin solid films by plasma-enhanced chemical vapor deposition (PECVD) is developed. The electron density and temperature have been evaluated through the ambipolar theory, assuming the Druyvesteyn function as the electron energy distribution function. All the model parameters related to both reaction and mass transport are evaluated a priori. Gas-phase reaction rates have been estimated by means of thermochemical methods. The model predictions are satisfactorily compared with experimental values of deposition rate and composition of the film.