Abstract: A semiconductor laser including an n-type GaAs substrate, an n-type clad layer formed over the substrate, an undoped AlGaAs active layer formed over the clad layer, the active layer having a varying aluminum content depending on the level therein, to form predetermined asymmetric quantum wells, a p-type clad layer formed over the active layer, and a p-type cap layer formed over the p-type clad layer. The active layer has a lower surface constituted by Al.sub.y Ga.sub.1-y As having a low aluminum content and an upper surface constituted by Al.sub.z Ga.sub.1-z As having a high aluminum content, and the aluminum content of the AlGaAs of the active layer present between the lower and upper surfaces increases gradually, while the AlGaAs of both types of the clad layers includes Al.sub.x Ga.sub.1-x As having an aluminum content higher than that of the active layer, to form an asymmetric graded quantum well structure. The AlGaAs of the active layer includes Al.sub.y Ga.sub.