  We explore a regime of unipolar electronic transport in a multiple quantum
well structure with very large current discontinuities - up to five orders of
magnitude. Magneto-transport experiments reveal different transport regimes.
Quantum well impact ionization shifts the structure from a resistive down
state, where the current flows through inter-well quantum tunneling, to a
highly conductive up state. In the latter regime, the current leaks through a
barrier suddenly broken down because of an efficient ionization of the first
quantum well.
