Comparison of Wz-phase and Znb-phase GaN IMPATT's with Si was investigated. GaN-based IMPATT's at D-band and potential of noise characteristics of Wz-phase and Znb-phase GaN IMPATT's was also analyzed. The noise of GaN-based IMPATT's was found to be equivalent to Si-based IMPATT's but was higher with respect to GaAs-based IMPATT's. The noise was found to decrease with increasign operating temperature.