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 first multiple quantum well structure containing aluminum and a plurality of semiconductor stacks. Each of the semiconductor stacks is stacked by a well layer and a barrier layer. The light-emitting structure emits an incoherent light. The well layer and the barrier layer in each of the semiconductor stacks include the same quaternary semiconductor material which includes indium (In). The well layer has a first In content percentage larger than 0.53, and the barrier layer has a second In content percentage less than 0.53.