In this paper the authors have proposed a simple method to study the change in the millimeter-wave performance of IMPATTs operating in MITATT mode from the shift of avalanche transit time (ATT) phase delay. A generalized double iterative computer method based on Gummel-Blue approach incorporating the effect of tunneling is used to obtain admittance characteristics and negative resistivity profiles of silicon based double drift region (DDR) device structures operating at different mm-wave frequencies in the presence of tunneling. It is observed that a shift of ATT phase delay occurs when tunneling is taken into account. This shift is obtained from the spatial variation of the negative resistivity profiles in the depletion layer of the device. The results show that the shift of ATT phase delay increases at higher operating frequencies of the device, which in turn leads to larger degradation of the mm-wave performance of the device at higher frequencies. © 2011 IEEE.