Ionization rate coefficients and saturation drift velocities for electrons and holes are the vital material parameters in determining the performance of an IMPATT diode.We have performed a sensitivity analysis of the millimeter wave characteristics of 4H-SiC and 6H-SiC IMPATT diodes with reference to the above mentioned material data and an operating frequency of 220 GHz.The effect of a small variation in the ionization rate and drift velocity on the device characteristics like break down voltage,efficiency,noise measure and power output has been presented here.