Abstract: A nitride semiconductor structure including a substrate, a cap layer, a nucleation layer, a transition layer and a composite buffer structure is provided. The cap layer is located on the substrate. The nucleation layer is located between the substrate and the cap layer. The transition layer is located between the nucleation layer and the cap layer, wherein the transition layer is an AlxGaN layer. The composite buffer structure is located between the transition layer and the cap layer. The composite buffer structure includes a first composite buffer layer, wherein the first composite buffer layer includes a plurality of first AlyGaN layers and a plurality of first GaN layers alternately stacking with each other, and the x is equal to the y.

Abstract: A nitride semiconductor structure is provided. The nitride semiconductor structure includes a substrate, a SiC nucleation layer, a composite buffer layer and a nitride semiconductor layer. The SiC nucleation layer is located on the substrate. The composite buffer layer is located on the SiC nucleation layer. The nitride semiconductor layer is located on the composite buffer layer. Besides, the nitride semiconductor structure is an AlN free semiconductor structure.