A semiconductor device comprises an active layer having a quantum well structure, the active layer including a well layer and a barrier layer and being sandwiched by a first conductivity type layer and a second conductivity type layer, wherein a first barrier layer is provided on side of the first conductivity type layer in the active layer and a second barrier layer is provided on the side of the second conductivity type layer in the active layer, at least one well layer is sandwiched thereby, and the second barrier layer has a band gap energy lower than that of the first barrier layer in the form of asymmetric barrier layer structure, where the second conductivity type layer preferably includes a carrier confinement layer having a band gap energy higher than that of the first barrier layer, resulting in a reverse structure in each of conductivity type layer in respect to the asymmetric structure of the active layer to provide a waveguide structure having excellent crystallinity and device characteristics in the nitride semiconductor light emitting device operating at a wavelength of 380 nm or shorter.