The temperature-dependences of ionized dopant concentration at different doping levels are generalized and the preconditions for thermal instabilities due to self-heating are studied. The nonisothermal simulations of forward-biased SiC structures over a wide temperature range are performed by using the drift-diffusion 1D-simulator DYNAMIT. Results show that the incomplete doping ionization will be an important effect if impurity activation energies exceed 0.1, 0.2 and 0.3 eV for doping levels 1019, 1018 and 1017cm−3, respectively. For appearance of S-shaped selfheated I/V curves the respective values must exceed 0.2, 0.3 and 0.4 eV. Strong influence of incomplete dopant ionization on forward I/V curves of realistic 4H-SiC and 6H-SiC p-i-n structures is predicted by simulations. At that the dominating role of the thick substrate layer is shown.