A self-consistent microscopic model of sequential tunneling in superlattices is employed to investigate self-sustained current oscillations. Current spikes - high-frequency modulation due to well-to-well hopping of charge monopole domain walls - are naturally reproduced. Moreover, as the contact doping shrinks, the recycling and motion of charge dipoles resulting in a lower-frequency oscillatory mode is predicted. For low contact doping, this mode dominates and monopole oscillations disappear. At intermediate doping, hysteresis between both the oscillatory modes may be possible.

This work has been supported by the DGES (Spain) grants PB97-0088 and PB96-0875, by the European Union TMR contracts ERB FMBX-CT97-0157 and FMRX-CT98-0180 and by the Community of Madrid, project 07N=0026=1998

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