This work presents the growth of quaternary barrier InAlGaN/GaN HEMT structures with in-situ SiN passivation by MOVPE. Five heterostructures with different SiN thicknesses, ranging from 0 to 22 nm, were realized. We observed that the growth of SiN onto the InAlGaN barrier results in smooth films, without formation of extended defects and without degradation of the structural properties of the barrier layer, even in the case of a SIN growth times as long as 90 min. In agreement with these findings, the sheet resistance of the heterostructures was found to be independent on the SiN deposition time, proving also that the SiN layer does not introduce additional strain into the heterostructure. The passivation effect of the SiN cap was demonstrated through Hall measurements. A significant increase of the sheet resistance after ohmic contacts realization was observed on the sample without SiN in-situ passivation and related to surface charges. This change was not observed on the HEMT structures with in-situ passivation, regardless the SiN layer thickness, showing that the SiN cap is an effective way to reduce the density of electronically active states at the interface between the III-N layers and the passivation. In addition, the structures show state of the art transport properties with 2DEG densities of 1.6x10(13) cm(-2) and electron mobilities as high as 1800 cm(2) V-2 s(-1).