A magnetic-field-dependent resistance change of eight orders of magnitude is obsd. in B-doped Si-SiO2-Al structures. In order to identify the elementary mechanisms governing this phenomenon, the thickness of the oxidic layer, which is used as an interface energy barrier, was varied by changing the exposure time to an O plasma. Next, the chem. compn. was monitored by in situ XPS measurements. From current-voltage measurements, we observe that at low temps., an ultrathin SiO2 layer provides the kinetic energy to trigger an autocatalytic process of impact ionization. A magnetic field suppresses the onset of impact ionization to higher elec. fields, resulting in a large magnetoresistance.