A gallium nitride-based semiconductor device includes a composite substrate and a gallium nitride layer. The composite substrate includes a silicon substrate and a filler. The silicon substrate includes a first surface and a second surface opposite to the first surface, and the first surface defines a number of grooves therein. The filler is filled into the number of grooves on the first surface of the silicon substrate. A thermal expansion coefficient of the filler is bigger than that of the silicon substrate. The gallium nitride layer is formed on the second surface of the silicon substrate.