A thermionic or thermotunneling generator or heat pump is disclosed, comprising electrodes substantially facing one another and separated by spacers disposed between the electrodes, wherein the substrate material for the cathode is preferably a single crystalline silicon wafer while the substrate for the anode is an organic wafer, and preferably a polished polyimide (PI) wafer. On the cathode side, standard silicon wafer processes create the 10-1000 nm thin spacers and edge seals from thermally grown oxide. Either wafer is partially covered with a thin film of material that is characterized by high electrical conductivity and low work function. In one embodiment, the cathode is partially covered with a thin film of Ag—Cs—O. In another embodiment, the anode is additionally covered with a thin film of Ag—Cs—O, in which case the work function of the cathode coating material is reduced further utilizing an Avto Metal structure of nanoscale patterned indents. A method for fabricating the composite structure device is further disclosed.