The Physics of Failure of MIS Devices Under Radiation

Unlike the solar cell and the NPN transistor, the MOS device does not sustain a degradation as the principal effect of exposure to nuclear radiation. Instead, the MOS device undergoes a change of operating region, the change being in the nature of a parallel shift of the characteristic curve of the device, produced by the trapping of radiation-excited holes within the 2000-Å insulator and the consequent buildup of a fixed bulk space charge in the insulator. Less significant changes under radiation are variations in the shape of the characteristic curve and increased leakage current. These are genuine degradation effects and are closely analogous to the strong effects of ionizing radiation in planar-passivated junction devices such as bipolar transistors, SCRs, diodes, etc. In the latter cases, the devices are acting as MIS devices and hole trapping in the oxide is again responsible for their degradation. A consideration of the case of simple MIS devices under radiation is thus found helpful in elucidating some other important types of failure under radiation of silicon junction devices.