We report on the effect of reactant gas stoichiometry of in-situ SiNx passivation on structural properties of MOCVD grown AlGaN/GaN high electron mobility transistor structures on 100 mm Si (1 1 1). A systematic study on the effect of constituent gas flows on surface morphology and growth rate is reported. X-ray reflectometry and atomic force microscopy is performed to determine the surface morphology and thickness of the near-surface layers. Conformal coverage of SiNx with abrupt SiN x -III-Nitride interface is confirmed by transmission electron microscopy. When the growth rate of the in-situ SiNx is less than a critical threshold of 10 nm/h, the AlGaN barrier layer is significantly etched. The charge density of the two-dimensional electron gas induced at the AlGaN/GaN interface due to polarization and surface state filling is evaluated with Hg-probe C-V profile and calculations based on strain relaxation. Increased sheet charge density is observed for the in-situ passivated samples. Passivated samples with growth rates higher than the critical threshold show excellent suppression of strain relaxation in the barrier layer. In addition to the strain induced carrier density, surface state filling with in-situ SiNx passivation contributes 8-12% of the total sheet charge density.