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Herein, 2D electron gas (2DEG) enhancement in an AlN/GaN/AlN double‐heterojunction high‐electron‐mobility transistor (DH‐HEMT) is achieved by epilayer stress engineering. The epistructures are grown on a SiC substrate using plasma‐assisted molecular beam epitaxy (PA‐MBE). The stress in the AlN buffer is systematically studied as a function of the III/V ratio. An optimized AlN buffer layer with the relaxation of 66% and a smooth surface morphology with a root mean square (RMS) roughness of 0.5 nm is grown using the two‐step growth method with recovery via metal consumption. It is observed that the stress in the GaN channel is influenced by the stress in the AlN buffer nonlinearly. Furthermore, the carrier mobility increases as the stress in the GaN channel is increased. However, the 2DEG density remains unchanged. An optimized AlN/GaN/AlN DH‐HEMT epistructure with very high 2DEG sheet carrier density of 4.1 × 1013 cm−2 and carrier mobility of 613 cm2  V−1 s−1 is achieved, which leads to higher device output power density.

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Over the last couple of decades, GaN-based materials have emerged as promising candidates for high power and high-frequency devices. This can be attributed to their unique and attractive properties such as wide range of bandgap, high saturation velocity, spontaneous and piezoelectric polarization, and high thermal and chemical stability. The AlGaN/GaN high