Self-heating of high-voltage (10 kV class) 4H-SiC rectifier p(+)-n-n(+) diodes has been studied experimentally and theoretically in the dc mode. An analytical model is suggested, which allows calculation of non-isothermal current-voltage (I-V) characteristics in the dc mode from the known isothermal I-V characteristics. It is shown that, at the conventional substrate width (300-3 50 mu m) and relatively small diode diameter (400 mu m), the main contribution to the total thermal resistance comes from the thermal resistance of the substrate. It is noted that the contribution to the total thermal resistance from the blocking base, substrate, and interlayer between the substrate and an external heat sink is inversely proportional to the squared structure diameter a 2. At the same time, the contribution from the external heat sink is inversely proportional to a. Hence, the relative contribution from the external heat sink increases with the working area, and at the common diameter of power diodes equal to 2-3 mm, just the contribution form external heat sink may be the most important, especially at a comparatively thin substrate. The contact resistance and its temperature dependence contribute appreciably to dc characteristics. (c) 2007 Elsevier Ltd. All rights reserved.