kage-related alpha-particle sources [304]. II-55 FUTURE TRENDS One of the biggest worries for single-event effects is how technology trends will impact device susceptibility in the future. Effects such as SEU have already been getting worse with technology evolution. In this section, we discuss key technology drivers, how technology trends may affect hardening strategies, and the phenomenon of single-event effects in ground-based and aircraft microelectronics, a topic of growing concern. Technology Drivers Impacting Single-Event Effects Technology parameters that influence the SEU sensitivity include gate length, gate oxide thickness, and power supply voltage. Increased SEU susceptibility as gate length is decreased has been well documented, and may be due to direct channel conduction [66], bipolar coupling mechanisms between the source and drain [70], or decreased gate capacitance as the gate area is reduced [260]. Figure 6.1 shows an example of the decreased threshold for SEU for two strike locations as gate length is decreased and all other parameters are held constant [70]. The strike locations are the center of the drain for the outside-the-well "off" transistor and the inside-the- well "off" transistor (panels (a) and (c) in Figure 3.10). The results shown were obtained using 3D mixed-level simulations of a model p-substrate technology with 150 kQ feedback resistors. As gate length is decreased, the SEU threshold for outside-the-well "off" strikes decreases slightly, and the threshold for inside-the-well "off" strikes decreases rapidly due to the bipolar effect [70]. As the power supply is decreased and all other parameters are held constant, the SEU threshold again decreases due to reduced current drive in the restoring transistor and less stored Outside the well OFF Inside the well OFF Gate Length (um) Figure 6.1. Scaling trend for SEU threshold LET as a function of gate length from simulations of a p-substrate technology with feedback resistors [70]. All other technology parameters were held constant. II