We have calculated transport properties of unintentionally doped n-type AlGaN/GaN heterostructures. Using a thermodynamic model of defect formation, we have modeled the charge transfer process in such heterostructures, obtaining good agreement with experiment. The large polarization fields in the heterostructure dramatically lower the formation energy of the surface defects, leading to the observed extremely large two-dimensional electron gas concentrations. Calculations of the low temperature mobilities were also performed, showing that alloy disorder and, in some cases, interface roughness, are the dominant low-temperature carrier scattering mechanisms. At low temperatures a maximum intrinsic mobility of about 105 cm2/V is predicted for these heterostructures.