An object of the present invention is to provide a semiconductor device capable of reducing the on-voltage and a manufacturing method thereof. According to the present invention, a semiconductor device includes a Si substrate, a p-type anode layer provided on the front surface of the Si substrate, an anode electrode provided on the p-type anode layer, an n-type cathode layer and a p-type cathode layer provided adjacent to each other on a back surface of the Si substrate, an Al alloy layer provided on the n-type cathode layer and containing Si, and an Al alloy layer provided on the p-type cathode layer and containing Si, in which impurity concentration in the n-type cathode layer is 1E19 cm−3 or higher and impurity concentration in the p-type cathode layer is 10% or lower of the impurity concentration in the n-type cathode layer.