A semiconductor substrate contains a clathrate compound of the following General Formula (I). The semiconductor substrate includes a variable-composition layer which includes a pn junction and where composition of the clathrate compound varies along a thickness direction. A rate of change in y in the thickness direction of at least a portion of the variable-composition layer is 1×10−4/μm or more. AxByC46-y   (I) In General Formula (I), A represents at least one element selected from the group consisting of Ba, Na, Sr, and K, B represents at least one element selected from the group consisting of Au, Ag, Cu, Ni, and Al, and C represents at least one element selected from the group consisting of Si, Ge, and Sn, x is 7 to 9, and y is 3.5 to 6 or 11 to 17.