A semiconductor device comprises: a first semiconductor structure; a second semiconductor structure on the first semiconductor structure; an active region between the first semiconductor structure and the second semiconductor structure, wherein the active region comprises multiple alternating well layers and first barrier layers, wherein each of the first barrier layers has a band gap, the active region further comprises an upper surface facing the second semiconductor structure and a bottom surface opposite the upper surface; a first electron blocking layer between the second semiconductor structure and the active region, wherein the first electron blocking layer having a band gap greater than the band gap of one of the first barrier layers; a first aluminum-containing layer between the first electron blocking layer and the active region, wherein the first aluminum-containing layer has a first thickness and a band gap greater than the band gap of the first electron blocking layer; and a second aluminum-containing layer on a side of the first electron blocking layer opposite to the first aluminum-containing layer, wherein the second aluminum-containing layer has a second thickness and a band gap greater than the band gap of the first electron blocking layer; and wherein a ratio of the second thickness of the second aluminum-containing layer to the first thickness of the first aluminum-containing layer is between 0.8 and 1.2.