High electron mobility transistor material and device properties based on AlxGa1−xN/GaN heterostructures with an Al-content ranging from 12 to 35% are presented. The Al-content of the low pressure metal-organic vapor-phase epitaxy grown samples on semi-insulating SiC and sapphire was measured by high resolution X-ray diffraction; carrier concentration and mobility by Hall effect. The sheet carrier density in the nominally undoped structures increases with the Al-content from 3.6 × 1012 to 1.4 × 1013 cm−2 due to spontaneous and piezoelectric polarization. Comparison of the electrical data with a model based on polarization and strain induced effects in dependence of the Al-composition shows good agreement. Further discussion and presentation on devices is restricted to data on SiC substrates. We quantitatively relate threshold voltage and extrinsic transconductance of FETs to the carrier concentration und thus to the Al-content in the barrier. Excellent large signal performance is achieved over the AlxGa1−xN-composition range from 14 to 30%.