Computer designs of flat profile single drift region (SDR) Impatt devices using Si1-xGex/Si heterostructure at W-band frequencies for x=10% and x=30%, respectively, where x is the Ge mole fraction, are presented. This study clearly shows that Ge mole fraction can suitably be chosen so that optimum performance of heterostructure Impatts as regards both power output and DC to RF conversion efficiency are realized.