In the present work, a Si (111) substrate has been patterned with 120‐μm wide square mesas separated by 5‐μm deep grooves. An Al0.29Ga0.71N/GaN HEMT structure was grown by molecular beam epitaxy with a 2‐μm thick Al0.15Ga0.85N buffer on a strain‐mitigating stack previously developed for GaN and low Al‐content AlGaN buffers. Surface inspections confirm the absence of cracks and roughness modification compared to planar growth. Interestingly, contrary to planar growth, X‐ray diffraction shows that plastic strain relaxation was inhibited within the 150‐nm GaN channel grown on top of the Al0.15Ga0.85N buffer. Devices were fabricated in order to assess the electrical properties of the grown films. C–V and TLM measurements reveal the presence of a 2DEG with a density of 7 × 1012 cm−2 and a sheet resistance around 450 Ω □−1. Round geometry transistors were measured up to 200–V drain bias. Compared to devices previously fabricated on planar structures with Al0.05Ga0.95N buffers, an order of magnitude lower off‐state leakage currents were obtained thanks to the larger Al content in the buffer.