A semiconductor device comprising: a first semiconductor region (2, 12, 22, 100) of a first conductivity type; and a second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) of a second conductivity type, having a different band gap from the first semiconductor region (2, 12, 22, 100) and forming a heterojunction with the first semiconductor region (2, 12, 22, 100), an electric field relaxation region (35, 61) of the second conductivity type with impurities introduced therein, formed in the first semiconductor region (2, 12, 22, 100) in contact with the heterojunction; and a punch-through preventive region (34, 50) of the second conductivity type formed in the first semiconductor region (2, 12, 22, 100) in contact with a junction face between the first semiconductor region (2, 12, 22, 100) and the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101), an impurity concentration of the punch-through preventive region (34, 50) being equal to or greater than an impurity concentration of the electric field relaxation region (35, 61), wherein a principal ingredient of the first semiconductor region (2, 12, 22, 100) is silicon carbide, wherein the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) is made of a material selected from the group consisting of monocrystalline silicon, amorphous silicon, and polysilicon, and wherein a thickness and an impurity concentration of the first semiconductor region (2, 12, 22, 100) is set so that a ratio of a potential distributed in the first semiconductor region (2, 12, 22, 100) to a potential in the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) when a reverse bias is applied to the heterojunction is in excess of a critical field strength ratio of the first semiconductor region (2, 12, 22, 100) to the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101), so that the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) includes a non-depletion area that is not depleted when a reverse bias is applied to the heterodiode, and so that the thickness of the non-depletion area that is not depleted when a reverse bias is applied to the heterodiode is greater than a diffusion length in the non-depletion area of carriers that act as majority carriers in the first semiconductor region (2, 12, 22, 100). a first semiconductor region (2, 12, 22, 100) of a first conductivity type; and a second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) of a second conductivity type, having a different band gap from the first semiconductor region (2, 12, 22, 100) and forming a heterojunction with the first semiconductor region (2, 12, 22, 100), an electric field relaxation region (35, 61) of the second conductivity type with impurities introduced therein, formed in the first semiconductor region (2, 12, 22, 100) in contact with the heterojunction; and a punch-through preventive region (34, 50) of the second conductivity type formed in the first semiconductor region (2, 12, 22, 100) in contact with a junction face between the first semiconductor region (2, 12, 22, 100) and the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101), an impurity concentration of the punch-through preventive region (34, 50) being equal to or greater than an impurity concentration of the electric field relaxation region (35, 61), wherein a principal ingredient of the first semiconductor region (2, 12, 22, 100) is silicon carbide, wherein the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) is made of a material selected from the group consisting of monocrystalline silicon, amorphous silicon, and polysilicon, and wherein a thickness and an impurity concentration of the first semiconductor region (2, 12, 22, 100) is set so that a ratio of a potential distributed in the first semiconductor region (2, 12, 22, 100) to a potential in the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) when a reverse bias is applied to the heterojunction is in excess of a critical field strength ratio of the first semiconductor region (2, 12, 22, 100) to the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101), so that the second semiconductor region (3, 7, 10, 19, 29, 40, 59, 63, 73, 89, 101) includes a non-depletion area that is not depleted when a reverse bias is applied to the heterodiode, and so that the thickness of the non-depletion area that is not depleted when a reverse bias is applied to the heterodiode is greater than a diffusion length in the non-depletion area of carriers that act as majority carriers in the first semiconductor region (2, 12, 22, 100).