Thermal diodes implemented in InSb with different emitter layers have been constructed and tested in order to understand better the physical mechanism associated with the observed enhancement of the figure of merit. In this study, we used a 1 mm thick solid InSb gap with a metal emitter, and a thin (few thousand Angstrom) p-type region to isolate the emitter from the solid gap region. Devices with different acceptor densities were studied. It was found that if the p-type doping was too low, that no enhancement over thermoelectric performance is observed. Over a relatively narrow range of acceptor doping concentration, a significant enhancement over thermoelectric performance is observed. The largest enhancement of the product of short-circuit current and open-circuit voltage observed was a fact of about 6. If the acceptor concentration is too great, the device performance is degraded.