A possible minimum toy model with negative differential capacitance for   self-sustained current oscillation
  We generalize a simple model for superlattices to include the effect of differential capacitance. It is shown that the model always has a stable steady-state solution (SSS) if all differential capacitances are positive. On the other hand, when negative differential capacitance is included, the model can have no stable SSS and be in a self-sustained current oscillation behavior. Therefore, we find a possible minimum toy model with both negative differential resistance and negative differential capacitance which can include the phenomena of both self-sustained current oscillation and $I-V$ oscillation of stable SSSs. 
Gang Xiong, Z. Z. Sun, X. R. Wang