4H-SiC P+NN+ structures have been fabricated following Medici(TM) software simulation in order to block voltages as high as 6 kV. In particular, these diodes are realized by surrounding the emitter by a Aluminum-implanted ring called Junction Termination Extension (JTE). Electrical characterizations under reverse bias at room temperature and in various environments (air, SF6) show a premature breakdown of the diodes. This breakdown is localized at the emitter periphery. OBIC (Optical Beam Induced Current) measurements show a peak of photocurrent at the emitter junction edge, indicating the presence of a high electric field. These results involve an effectiveness of 60 % of the JTE. This is probably related to a low electrical activation of the implanted aluminum during the post-implantation annealing and to the presence of positive charges at the surface of the devices.