The change of both microwave negative resistance (R) and its parasitic series resistance (R-s) on the rise of junction temperature in the range of 100-220 degrees C of HP n(++)np(++) Si IMPATT [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Dev. 40 (1993) 1890] diode at X band (8-12 GHz) have been simulated. The studies followed by Gummel-Blue Technique [H.K. Gummel, J.L. Blue, IEEE Trans. Electron. Dev. 14 (1967) 569] show that for a constant experimental bias current of 25 mA [Mitra et al., 1993], for which the space charge effect is not prominent, the values of negative conductance and negative resistance degrade taking into account the changes in the ionization rates and drift velocities due to rise of temperature. Also observed that the critical series resistance increases with the increase of temperature up to 2.23 Omega, slightly higher than the realistic limit of 2 Omega [Mitra et al., 1993]. (c) 2006 Elsevier B.V. All rights reserved.