An optoelectronic semiconductor device includes a substrate, a first type semiconductor structure, a second type semiconductor structure, an active structure and a contact structure. The first type semiconductor structure is located on the substrate and has a first protrusion part with a first thickness and a platform part with a second thickness. The second type semiconductor structure is located on the first type semiconductor structure. The active structure is between the first type semiconductor structure and the second type semiconductor structure. The contact structure is disposed between the first type semiconductor structure and the substrate. The second thickness of the platform part is in a range of 0.01 μm to 1 μm.