A light emitting device comprising: a first conductivity type semiconductor layer; a second conductivity type semiconductor layer on the first conductivity type semiconductor layer; and an active layer (121; 121A; 121B) between the first and second conductivity type semiconductor layers (117, 125), the active layer including a plurality of well layers and a plurality of barrier layers, wherein the plurality of well layers comprise a first well layer and a second well layer adjacent to the first well layer, wherein the plurality of barrier layers comprise a first barrier layer disposed between the first and second well layers (W1, W2), wherein the first barrier layer comprises a plurality of semiconductor layers (33, 34, 35; 33A, 34A, 35A) having an energy bandgap wider than an energy bandgap of the first well layer, and wherein at least two layers of the plurality of semiconductor layers which is adjacent to the first and second well layers (W1, W2) have aluminum, Al, contents greater than that of the other layer, wherein the plurality of semiconductor layers comprise: a first semiconductor layer (33; 33A) having a second energy bandgap wider than a first energy bandgap of the first well layer and having AIGaN; a second semiconductor layer (34; 34A) between the first well layer and the first semiconductor layer (33, 33A), the second semiconductor layer (34; 34A) having a third energy bandgap wider than the second energy bandgap and including AlN material; and a third semiconductor layer (35; 35A) between the first semiconductor layer (33; 33A) and the second well layers, the third semiconductor layer (35; 35A) having a fourth energy bandgap wider than the second energy bandgap and including AlN material, wherein an Al content of the first semiconductor layer (33; 33A) is in the range of 5% to 30%, wherein the second and third semiconductor layers (34, 35; 34A, 35A) have lattice constants smaller than that of the first semiconductor layer (33; 33A), wherein the second and third semiconductor layers (34, 35; 34A, 35A) have an Al composition greater than that of the first semiconductor layer (33; 33A), wherein the first conductivity type semiconductor layer includes AIGaN, wherein the second conductivity type semiconductor layer includes AIGaN, and wherein the active layer (121; 121A; 121B) emits a UV light having a peak wavelength of 285 nm to 385 nm. a first conductivity type semiconductor layer; a second conductivity type semiconductor layer on the first conductivity type semiconductor layer; and an active layer (121; 121A; 121B) between the first and second conductivity type semiconductor layers (117, 125), the active layer including a plurality of well layers and a plurality of barrier layers, wherein the plurality of well layers comprise a first well layer and a second well layer adjacent to the first well layer, wherein the plurality of barrier layers comprise a first barrier layer disposed between the first and second well layers (W1, W2), wherein the first barrier layer comprises a plurality of semiconductor layers (33, 34, 35; 33A, 34A, 35A) having an energy bandgap wider than an energy bandgap of the first well layer, and wherein at least two layers of the plurality of semiconductor layers which is adjacent to the first and second well layers (W1, W2) have aluminum, Al, contents greater than that of the other layer, wherein the plurality of semiconductor layers comprise: a first semiconductor layer (33; 33A) having a second energy bandgap wider than a first energy bandgap of the first well layer and having AIGaN; a second semiconductor layer (34; 34A) between the first well layer and the first semiconductor layer (33, 33A), the second semiconductor layer (34; 34A) having a third energy bandgap wider than the second energy bandgap and including AlN material; and a third semiconductor layer (35; 35A) between the first semiconductor layer (33; 33A) and the second well layers, the third semiconductor layer (35; 35A) having a fourth energy bandgap wider than the second energy bandgap and including AlN material, a first semiconductor layer (33; 33A) having a second energy bandgap wider than a first energy bandgap of the first well layer and having AIGaN; a second semiconductor layer (34; 34A) between the first well layer and the first semiconductor layer (33, 33A), the second semiconductor layer (34; 34A) having a third energy bandgap wider than the second energy bandgap and including AlN material; and a third semiconductor layer (35; 35A) between the first semiconductor layer (33; 33A) and the second well layers, the third semiconductor layer (35; 35A) having a fourth energy bandgap wider than the second energy bandgap and including AlN material, wherein an Al content of the first semiconductor layer (33; 33A) is in the range of 5% to 30%, wherein the second and third semiconductor layers (34, 35; 34A, 35A) have lattice constants smaller than that of the first semiconductor layer (33; 33A), wherein the second and third semiconductor layers (34, 35; 34A, 35A) have an Al composition greater than that of the first semiconductor layer (33; 33A), wherein the first conductivity type semiconductor layer includes AIGaN, wherein the second conductivity type semiconductor layer includes AIGaN, and wherein the active layer (121; 121A; 121B) emits a UV light having a peak wavelength of 285 nm to 385 nm.