e boundary [61]. Funneling affects the rate at which the charge deposited in the epi layer is collected, but does not appear to change the total charge collection, since this charge would usually be collected even in the absence of funneling. While in some cases important to charge collection in isolated p/n junctions with constant applied bias, the role of the funnel is less significant in the case of static circuits such as SRAMs, where reverse-biased transistor junctions are connected to active external circuitry. In this scenario, the applied voltage at the struck junction is not constant, and in fact very often the n+ Region n+ Region 3.0 V 2.0 V Figure 3.3. Illustration of funneling in an n+/p silicon junction following an ion strike: a) electrostatic potential, b) electron concentration. Note that contours of electrostatic potential are distorted along the path of the ion [53]. II-14 diode n+ -p-p diode t (ns) Figure 3.4. Charge-collection transients in n+/p and p+/n epitaxial diodes following ion strikes [61]. Note the lack of a knee in the p+/n diode curve, which indicates that funneling is very weak for this case. struck node may switch from being reverse-biased to zero-biased (see Section 3.4). This loss of bias at the struck node tends to lessen the importance of drift collection (and hence the funnel) as the single-event transient proceeds [63]. In such cases, funneling may play a role in the early-time response of the circuit by helping initially flip the node voltage, but it is late-time collection by diffusion that ensures the bit stays flipped (see Section 3.4 for further details about the upset process in SRAM circuits). 3.2.2 New Charge-Collection Mechanisms in Submicron Devices The charge-collection response of a single p/n junction is generally presumed to accurately depict the response of the sensitive junction of a transistor, typically a reverse-biased drain region. Studies have indicated that a new charge-collection mechanism may exist for submicron MOS transistors which requires consider