Semiconductor structure and method for forming the same

A semiconductor structure is provided. The semiconductor structure includes a substrate having a front side and a back side opposite the front side. The semiconductor structure also includes a first contact metal layer disposed on the front side of the substrate. The semiconductor structure further includes a III-V compound semiconductor layer disposed between the substrate and the first contact metal layer. Moreover, the semiconductor structure includes a via hole penetrating through the substrate and the III-V compound semiconductor layer from the back side of the substrate. The bottom of the via hole is defined by the first contact metal layer, and the first contact metal layer includes molybdenum, tungsten, iridium, palladium, platinum, cobalt, ruthenium, osmium, rhodium, rhenium, or a combination thereof.