A semiconductor laser having such a structure that a laser structure including a laser active region of multi quantum well structure, cladding layers and a cap layer is formed on a substrate, is disclosed in which both end regions of the laser active region in the quantum well layer is converted into mixed crystal by impurity induced intermixing so that a multi quantum well active region is sandwiched between mixed crystal regions, and impurity diffused regions are formed between the surface of the crystal and the mixed crystal regions, to form a current path and to inject carriers into the multi quantum well region in a direction parallel to the laser active layer. Thus, the semiconductor laser can modulate laser oscillation at a very high frequency, and moreover is readily fabricated or integrated.