We investigate the influence of the junction potential distribution on the effective impurity time constants in SiC and find their deviation from the values predicted by the traditional admittance spectroscopy analysis. The impurity time constant behavior is found as a function of impurity parameters, such as activation energy and concentration as well as experimental conditions, such as temperature and measurement ac signal amplitude. The corrected values of the impurity time constant are applied for Arrhenius plot analysis for N-doped 6H-SiC.