Boron diffusion can be used to compensate the n-type layer of a p(+)nn(+) 6H-silicon carbide structure in order to increase its high-voltage capabilities. Measurements under reverse biases for a current range from 10 to 500 mu A show that this process is very efficient for working temperatures about 300 K. Indeed we obtained a voltage of 670 V for a reverse current of 10 mu A instead of the 120 V calculated for a structure without boron diffusion. Nevertheless, the breakdown voltage decreases rapidly when the temperature increases. Capacitance measurements show that the measured doping level in the n-type layer evolves in the same way as the temperature (it ranges from 10(13) cm(-3) at 300 K to 10(17) cm(-3) at 500 K). A great concentration of boron seems to be responsible for this doping variation with temperature. Admittance spectroscopy reveals the presence of D centers at 0.62 eV above the valence band associated to boron at concentration similar or superior to nitrogen concentration in the n-type layer. The increase of the doping level with the temperature is responsible for this decrease of the breakdown voltage.