A thermionic converter for converting thermal energy to electrical energy includes an emitter and a collector. The emitter emits thermionic electrons upon receipt of heat from a heat source. The emitter is made of a first semiconductor material to which a first semiconductor impurity is doped with a first concentration. The collector is spaced and opposite to the emitter to receive the thermionic electrons emitted from the emitter so that the thermal energy is converted to electrical energy. The collector is made of a second semiconductor material to which a second semiconductor impurity is doped with a second concentration less than the first concentration.