Abstract: A semiconductor stack includes a first semiconductor structure, a second semiconductor structure and an active structure. The active structure includes a first well set, a second well set and a plurality of barriers. The first well set is disposed on the first semiconductor structure and includes one or multiple first wells. The second well set is disposed between the first well set and the second semiconductor structure and includes one or multiple second wells. The plurality of barriers is arranged alternately with the one or multiple first wells and the one or multiple second wells. The first well has a first thickness. The second well has a second thickness different from the first thickness. The one or multiple first wells and the one or multiple second wells include AlxInyGa1?x?yN respectively, wherein 0?x?1, 0?y?1, 0?1?x?y<1.

Abstract: A semiconductor stack includes a first-type semiconductor layer, a second-type semiconductor layer, an active region located between the first-type semiconductor layer and the second-type semiconductor layer, one or multiple recesses, and a recess-induced layer located between the first-type semiconductor layer and the active region. The active region has a first thickness and includes an upper surface and a lower surface closer to the first-type semiconductor layer than the upper surface. Each recess includes a bottom disposed in the active region. A first distance is from the bottom of the recess to the lower surface. The first distance is 0.5-0.9 times the first thickness.

Abstract: A light-emitting device includes a first nitride semiconductor structure; a stress relief structure on the first nitride semiconductor structure including a plurality of narrow band gap layers and a plurality of wide band gap layers alternately stacked, wherein one of the plurality of wide band gap layers includes a plurality of wide band gap sub-layers and one of the plurality of wide band gap sub-layers includes aluminum; an active structure on the stress relief structure including a plurality of quantum well layers and a plurality of barrier layers alternately stacked, wherein one of the plurality of barrier layers includes a plurality of barrier sub-layers and one of the plurality of barrier sub-layers includes aluminum, an aluminum composition of the wide band gap sub-layer is greater than or equal to that of the barrier sub-layer, and an average aluminum composition of the wide band gap layer is greater than that of the barrier layer; and an electron blocking structure on the active structure.