Also, because a smaller, faster SRAM cell has lower capacitance, the feedback resistance required to harden the cell has to be raised to compensate. This can lead to a requirement for very large resistors in advanced technologies. This point is illustrated in Figure 6.4, which shows the results of 3D mixed-level simulations of two resistively-hardened SRAM technology generations. The simulated SEU threshold LET is plotted as a function of feedback resistor value for a 0.6-um, 5-V technology with transistor widths of II-58 2.3 um and 1.3 um, and for a 0.35-um, 3.3-V technology with a transistor width of 0.75 um. As can be seen from the predictions, achieving a target LET threshold of 40 MeV-cm2/mg would require a 200-kQ resistor for the 2.3x0.6 um 2 device, a 600-kQ resistor for the 1.3x0.6 um2 device, and a whopping 1.1-MO resistor for the 0.75x0.35 um2 device. Such large resistors would be very difficult to controllably manufacture and would show a significant temperature dependence [151]. Because of this problem and stagnant performance levels, it is likely that for applications requiring a high level of SEU hardness, devices below about 0.5-um will utilize other hardening techniques, such as SOI, active feedback elements, or circuit hardening. Terrestrial and High-Altitude Single-Event Effects To this point we have discussed the natural space radiation environment and how it interacts with microelectronics to cause single-event effects. A radiation environment also exists in the Earth's atmosphere, and although less harsh than the space environment, it can also give rise to SEE. In the following sections we will discuss the high-altitude and terrestrial radiation environments and their effects on microelectronics in aircraft electronics systems and at ground level. 6.3.1 The Atmospheric Radiation Environment The atmospheric radiation environment comes about as a result of the space radiation environment impinging on Earth's atmosphere. As very highly energetic cosmic rays enter the upper atmosphere they intera