main disadvantages of IMPATT diodes are their hi phase noise caused by the avalanche effect on which they operate, and they need hi voltage usually 70V +
They are higher power generators than Gunn diodes tho
Gunn Diodes are lower voltage operation usually less than 12V DC. Usually always less than 1W output typically 10 to 150mW output for a given diode.
becayse they operate differently to the IMPATT diode they generate much lower phase noise
Dave