Bipolar characteristics of AlGaN/AlN/GaN/AlGaN double heterojunction structure with AlGaN as buffer layer

•2DEG and 2DHG coexist in the AlGaN/AlN/GaN/AlGaN DH-structure.•The sheet densities of 2DEG and 2DHG vary with buffer Al content and GaN thickness.•The conditions for the disappearance of 2DHG are discussed.•Increasing buffer Al content provides better electron confinement.•Dislocation scattering is...

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