<p>We present a novel structure that incorporates a composite of superlattice AlGaN and three-dimensional (3D) GaN, which achieved high-quality crack-free GaN epitaxial films on a Si substrate. The superlattice structure plays a significant role in alleviating the lattice mismatch between GaN and Si. More importantly, the introduction of 3D GaN structure based on the superlattice structure delays the merging process of islands, improving the quality of GaN films and density of dislocations. This composite structure enhances crystal quality, alleviates the stress in GaN, and reduces in the density of dislocations. The composite structure also provides the possibility for growing a high-resistance buffer layer, which can be used to replace Fe doping to enhance the breakdown voltage (B<i>V</i>_gd). The significant reduction of dislocation density significantly improves the B<i>V</i>_gd of devices fabricated by epitaxy. Furthermore, the composite structure improves the current density and current collapse. This work presents a novel method to enhance the breakdown voltage of device.</p>