is illustrated in Figure 3.5a and is triggered by a particle strike that passes through both the source and the drain at near-grazing incidence. Immediately following the strike, the electrostatic potential in the channel region is perturbed to the extent that i) there is no longer a potential barrier between the source and channel, and ii) there is a substantial potential gradient between the source and the drain. These two conditions together can lead to a significant (but short-lived) source-drain conduction current which mimics the "on" state of the transistor. This mechanism was revealed by 3D alpha-particle simulations and has been experimentally verified. The experiments indicate that source charge injection due to the ALPEN mechanism increases rapidly II-15 W = 1.0 um O = 4 Vg=0V Vs = OV Vd = 5V Vds=5V funneling equipotential Vds=3V lines Leff ( um ) Figure 3.5. a) Illustration of the alpha-particle induced source-drain penetration (ALPEN) effect [64]. b) Charge injected by the source due to the ALPEN mechanism as a function of gate length. For gate lengths below 0.5 um, this mechanism may become critical. for effective gate lengths below about 0.5 um, as shown in Fig. 3.5b. Later work predicted the same direct channel conduction mechanism can occur in 0.3-um gate length MOSFETs even for normal incidence strikes, and can lead to charge multiplication [66]. This mechanism may forebode a serious vulnerability to SEU for deep submicron MOSFETs. A somewhat similar, but distinct mechanism exists when electrons or holes released by a particle strike are confined to a well or body region in which a transistor is located. For example, for an n-channel MOS transistor located in a p-well, electrons induced by a particle strike can be collected at either the drain/well junction or the well/substrate junction. However, holes left in the well raise the well potential and lower the source/well potential barrier, and the source injects electrons into the channel, as illustrated in Figure 3.6 [67-70]. These electrons can