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We propose and demonstrate an AlGaN/GaN/AlGaN double heterostructure (DH) with significantly improved two-dimensional (2D) confinement for high-power III-N heterostructurefield-effect transistors(HFETs). The DH was grown directly on an AlN buffer over i-SiC substrate. It enables an excellent confinement of the 2D gas and also does not suffer from the parasitic channel formation as experienced in past designs grown over GaN buffer layers. Elimination of the GaN buffer modifies the strain distribution in the DH, enabling Al contents in the barrier region well over 30%. For the AlGaN/GaN/AlGaN DH design, the 2D electron gasmobility achieved was 1150 cm2/V s at room temperature and 3400 cm2/V s at 77 K, whereas the temperature independent sheet carrier density was NS≈1.1×1013 cm−2. Compared to a regular AlGaN/GaN structure, the channel mobility-concentration profiling shows significant improvement in the carrier confinement. Sample DHFETs with 1-μm long gates demonstrate the threshold voltage of 3.5 V, with a peak saturation current of 0.6–0.8 A/mm.