A semiconductor device is provided, which includes a first semiconductor structure, a second semiconductor structure and a light-emitting structure. The light-emitting structure is located between the first semiconductor structure and the second semiconductor structure. The light-emitting structure includes a multiple quantum well structure. The multiple quantum well structure contains aluminum and includes a plurality of semiconductor stacks. Each of the semiconductor stacks is stacked by a well layer and a barrier layer. In each semiconductor stack, the well layer has a thickness larger than a thickness of the barrier layer.