A semiconductor device includes a substrate, and a first transistor disposed on the substrate. The first transistor includes a first channel layer, a magnesium oxide layer, a first gate electrode, a first gate dielectric and first source/drain electrodes. A crystal orientation of the first channel layer is <100> or <110>. The magnesium oxide layer is located below the first channel layer and in contact with the first channel layer. The first gate electrode is located over the first channel layer. The first gate dielectric is located in between the first channel layer and the first gate electrode. The first source/drain electrodes are disposed on the first channel layer.

Provided are a substrate for epitaxially growing a diamond crystal, having at least a surface made of a metal, in which the above surface made of the metal is a plane having an off angle of more than 0, and the full width at half maximum of the X-ray diffraction peak from the (002) plane by the X-ray rocking curve measurement at the above surface made of the metal is 300 seconds or less; and a method of manufacturing a diamond crystal, including epitaxially growing a diamond crystal on the above surface made of the metal of the above substrate.