an object of the present invention is to provide a lowcost semiconductor substrate made of an aluminum - silicon carbide composite material that has excellent thermal performance and that is capable of maintaining high dimensional accuracy and stability when practically used while coping with the flow of rapid diversification of a practical shape . another object is to provide a package that can mount a semiconductor element that uses the substrate , and provide a semiconductor device that uses the substrate . in an aluminum - silicon carbide semiconductor substrate whose first component is a metal chiefly composed of aluminum and whose second component is silicon carbide , the second component is compositionally 5 to 60 % by weight of the whole and the remainder is the first component , and a warp in the direction of its main surface is 3 μm / mm or less .