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The effect of barrier alloy composition, x, on two-dimensional electron density and mobility are calculated for AlxGa1-xN/GaN heterostructures, both unintentionally doped and modulation-doped. For modulation-doped structures the possible effect of strain relaxation is also considered. The results agree with published experimental curves in tendency, i.e., when x increases, electron densities increase and electron mobilities decrease. By analyzing the effects of various scattering processes, it is revealed that the tendency of mobility vs. x is primarily determined by the rise of electron densities, which is further demonstrated when modulation-doping and strain relaxation are considered. Channel conductance vs. x is primarily determined by the factor with greater change, that is, electron density. Strain relaxation works counter to modulation-doping, and, as a result, the former is unfavorable and the latter is favorable to improving channel conductance.