Following Gummel-Blue approach  [H.K. Gummel, J. L. Blue, IEEE Trans. Electron. Devices 14(1967) pp. 569-572.], the effect of undepleted epitaxial layers on the series resistance (Rs) as well as on its microwave properties of single drift region (n+np+) Si IMPATT diodes  [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Devices 40(1993) pp. 1890-1893.] with flat doping profile with capacity 0.2PF at X band under experimental bias current of 25 mA and temperature 373 K have been studied. The computation for series resistance fits well with the device data for flat doping profile  [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Devices 40(1993) pp. 1890-1893.]. The same study has also been simulated on its low-high-low (lhl) doping profile counterpart. The value of Rs increases approximately linearly with the increase of undepleted epi-layer thickness determined by the doping density for both flat and lhl structure. The value of Rs decreases remarkably as the doping profile changes from flat to lhl type. © 2006 Elsevier Ltd. All rights reserved.