In this paper, an attempt is made to investigate the millimeter-wave performance of anisotype Si/Si1-xGex heterojunction Double-drift region (DDR) Impact Avalanche Transit Time (IMPATT) devices operating at 94 GHz window frequency by using a generalized double-iterative computer method based on drift-diffusion model developed by the authors. Simulation study on both anisotype Np and nP type Si/Si1-xGex heterojunction DDR IMPATTs for Ge mole fractions, x = 0.1 and x = 0.3, is carried out and their mm-wave properties are compared with a Si homojunction DDR IMPATT operating at the same frequency. Results show that significant improvements in DC to RF conversion efficiency and RF power output can be achieved in Si/Si1-xGex heterojunction DDR IMPATTs as compared to their Si homojunction counterpart. It is observed that the mm-wave performance of nP Si0.7Ge0.3/Si heterojunction DDR as regards to RF power output and conversion efficiency is best among all the devices under consideration. DC to RF conversion efficiency and continuous wave (CW) RF power output of nP Si0 7Ge0 3/Si DDR are obtained as 20.04% and 731.7 mW, respectively, near 94 GHz; whereas for Si DDR, the same parameters are obtained as 10.40% and 609.3 mW, respectively. Copyright. © 2013 by the IETE.