A mount structure includes two members that are bonded to each other with a bonding material layer having a first interface layer and a second interface layer at the interfaces with the two members. The bonding material layer contains a first intermetallic compound and a stress relaxation material. The first intermetallic compound has a spherical, a columnar, or an oval spherical shape, and the same crystalline structure as the first interface layer and the second interface layer, and partly closes the space between the first interface layer and the second interface layer. The stress relaxation material contains tin as a main component, and fills around the first intermetallic compound.

A semiconductor device includes a semiconductor chip having first and second main electrodes disposed on opposite surfaces of a silicon carbide substrate, first and second heat dissipation members disposed so as to sandwich the semiconductor chip, and joining members disposed between the first main electrode and the first heat dissipation member and between the second main electrode and the second heat dissipation member. At least one of the joining members is made of a lead-free solder having an alloy composition that contains 3.2 to 3.8 mass % Ag, 0.6 to 0.8 mass % Cu, 0.01 to 0.2 mass % Ni, x mass % Sb, y mass % Bi, 0.001 to 0.3 mass % Co, 0.001 to 0.2 mass % P, and a balance of Sn, where x and y satisfy relational expressions of x+2y11 mass %, x+14y42 mass %, and x5.1 mass %.