It is well known that AlGaN/GaN-based transistors are highly suitable for high power and/or high frequency, robust operation in applications such as RF and mm-wave communications, power electronics and more. To achieve success in development of such devices it has been necessary to introduce steps to circumvent certain properties that are device-detrimental, some of which are not yet completely understood. One such issue is that of the use of dielectric layers. These have been introduced as passivation to combat the well-known issue of DC-RF current dispersion, for field plating, and for gate dielectrics to achieve enhancement-mode devices, necessary for power switching applications. For the most part, the effect of the introduced dielectric layers has been investigated with respect to overall transistor device performance, that is, macroscopic parameters such as drain current, rather than the effect on the 2DEG channel itself. However, assessing the changes induced in the transport properties of the 2DEG as a result of application of these layers is an important step in gaining a better understanding of the role of these layers in altering device performance. In this work, the influence of passivation with silicon nitride of different stress states on the 2DEG transport in Al<SUB>x</SUB>Ga<SUB>1−x</SUB>N/GaN heterostructures is presented for the first time as a function of varying Al mole fraction (x). Samples from four heterostructures with x = 0.15, 0.23, 0.29 and 0.35 were deposited with SiN<SUB>x</SUB> by plasma enhanced chemical vapour deposition under compressive, neutral (slightly compressive), and tensile stress conditions (stress was measured for the same films deposited on silicon wafers). For each piece (different compositions and different SiN<SUB>x</SUB> layers) Hall bars were fabricated and the transport properties were measured using a variable magnetic field Hall technique, at 25, 77, 150 and 300K, up to 12T magnetic field. All types of SiN<SUB>x</SUB> passivant induced an increase in 2DEG concentration, consistent with previous published results for SiN<SUB>x</SUB> passivation. In addition, however, the 2DEG mobility increased significantly after passivation for the sample with x = 0.15. The more tensile the film stress, the greater was the percentage increase in mobility. However this percentage change decreased with increasing Al mole fraction. It is evident that silicon nitride passivation of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N/GaN structures induces different changes in 2DEG carrier concentration and mobility, dependent on both stress in the SiN<SUB>x</SUB> thin film and the Al mole fraction. It is equally apparent that there is a complex relationship between these different factors, and that the stress in the SiNx passivation layer plays some, but not always the principal, role in determining the 2DEG transport properties. Thus tailoring of the deposition conditions to optimise transport properties is critical for a given passivant and Al mole fraction, to maximise device performance.