Scientists demonstrate double charge carrier type switching of tin SnSe semiconductor by doping of antimony Sb. The SnSe carrier type switches from p-type to n-type, and re-switches to p-type as doping increases, due to the switching of major Sb substitution site from Se to Sn, promising reliable charge polarity control, leading to realization of SnSe-based p/n homojunction thermoelectric device for converting waste heat into electricity and new insights on impurity doping of compound semiconductors.