re as a function of oxide thickness. Their results II-42 are shown in Figure 4.8, where the critical field has been normalized to the value at normal incidence. The results clearly show that the angular dependence of SEGR does indeed disappear for thin oxides. The fact that off-normal ion strikes are effective at producing SEGR increases the threat environment in the isotropic heavy ion environment of space, and this must be accounted for when estimating SEGR susceptibility for thin oxides [209]. The angular data have been explained in terms of ion track structure, and an analytical model for the angular dependence based on a conductive pipe mechanism has been proposed [209]. Single-Event Burnout Single-event burnout (SEB) due to heavy ions, neutrons, and protons has been observed in both vertical power MOSFETs and bipolar transistors [194,195,212-216]. Similar to single-event latchup, SEB is a destructive failure mechanism that comes about due to a parasitic bipolar transistor structure inherent to some devices. Looking again at the power MOSFET structure in Fig. 4.3, a parasitic bipolar transistor is formed by the n-source (emitter), p-body (base), and n- epitaxial (collector) regions. Following an ion strike, currents flowing in the p-body can forward bias the emitter-base junction of the parasitic BJT due to the finite conductivity of the p-body region. The parasitic BJT is now operating in the forward active regime, and if the drain-to- source voltage is higher than the breakdown voltage (BVCEO) of the parasitic BJT, avalanche multiplication of the BJT collector current can occur. If this positive feedback (regenerative) current is not limited, it can lead to junction heating and the eventual burnout of the device [212]. 283-MeV Br LET ~ 37 MeV-cm2/mg 1/cos(0) 19.2 nm Wrobel '87 18 nm 12 nm 1/cos(0) Figure 4.8. Angular dependence of the critical field to rupture for oxides of varying thickness [209]. The angular dependence disappears as the oxide thickness approaches the diameter of the 283-MeV bromine ion t