Disturbing high frequency oscillations observed during turn-off of IGBT modules can be explained by (i) a local parasitic resonant circuit and (ii) a new r.f. generator mechanism similar to the BARITT diode effect. The phenomenon is related to periodic hole extraction from the electron-hole plasma. Therefore it will be called "plasma extraction transit time" effect. This effect can be modelled using device simulation. The r.f. generation efficiency is very small. Resonator Q-factor, resonance frequency and carrier transit time must fit well to enable excitation of the parasitic resonant circuit. (C) 2002 Elsevier Science Ltd. All rights reserved.