Metal‐oxide‐silicon (MOS) capacitors used in the breakdown mode were constructed to detect252Cf fission fragments. The physical effects related to radiation‐induced breakdown were studied and found to have a relationship to intrinsic dielectric breakdown. Physical studies were made of the effect of oxide annealing temperature between 100 and 400 K, oxide thickness, differing metal electrodes, and the materials interaction due to the spark discharge breakdown. By means of both silicon‐ion and cesium‐ion implants near the metal‐SiO2interface of the MOS capacitor, the current emission into the SiO2was found to be significantly enhanced; however, the enhanced emission was found to have no effect on increasing the sensitivity to lower specific ionization radiation as seen by fission‐fragment detection. The results indicate that the radiation‐induced breakdown mechanism occurs within the bulk, or body, of the SiO2film and is not influenced by conditions at the dielectric interfaces. It was found that thin films of the refractory metals HfO2or ZrO2between the oxide film and a top aluminum electrode prevented Al‐SiO2interaction during discharge and allowed operation of the MOS capacitor at higher electric fields. Obtaining higher operating fields significantly improved the minimum specific ionization detectability to 14 MeV cm2/mg from a prior value of 22 MeV cm2/ mg.