The semiconductor device includes a first conductivity-type semiconductor structure comprising a first stack and a second stack, wherein the first stack comprises alternate first layers and second layers, the second stack comprises alternate third layers and fourth layers. The semiconductor device includes a second conductivity-type semiconductor structure on the first conductivity-type semiconductor and includes an active structure between the first conductivity-type semiconductor structure and the second conductivity-type semiconductor structure. The first stack is between the active structure and the second stack, and a first difference between a maximum of the first doping concentration of one of the first layers and a minimum of the second doping concentration of one of the second layers is less than a second difference between a maximum of the third doping concentration of one of the third layers and a minimum of the fourth doping concentration of one of the fourth layers.