This paper, Part I of two, presents the results of a study combining experimental and modeling approaches of low pressure chemical vapor deposition (LPCVD) of silicon nitride from a silane-ammonia mixture. The experimental study consists in a reduced number of runs, chosen in order to identify the main features of the deposition process, i.e., marked nonuniformities at the wafer edge both in thickness and in Si/N composition. It is then shown that a complex gas-phase mechanism may be responsible for the observed physicochemical phenomena. A gaseous reaction model is thus proposed for a silane-ammonia mixture under typical low pressure CVD conditions. A complete reaction scheme is first studied. A thorough quantum Rice Ramsberger Kassel (QRRK) analysis compensated for the lack of kinetic information in the gas phase and allowed the identification of kinetic constants for uni- and bimolecular reactions. Its appropriateness is examined with one-dimensional nonsteady computations. A combined analysis of these calculations and of the QRRK results shows that the reaction model could be simplified, thus leading to a reduced reaction set reproducing the essential features of the full mechanism experimentally observed, which involves six species with two silylamine intermediates SiH3NH2 and SiHNH2. In Part II of this article series, this mechanism is integrated in a 2-D model of LPCVD reactors, previously developed in the laboratory (called CVD2) and adapted to this kind of deposition, taking into account hydrodynamics, mass transport, and chemical reactions.