Collection of Charge on Junction Nodes from Ion Tracks

TLDR

Charge collection from ion tracks can upset logic and memory in high‑density ICs, with drift conduction dominating when tracks penetrate junctions; earlier work focused on diffusion, but recent studies emphasize drift via a funnel that channels charge from the substrate. The study aims to derive an approximate analytic solution for ion‑track charge collection to aid circuit and system modeling. The authors model the funneling phenomenon with two‑dimensional simulations and present an analytic expression I(t)=I₀[exp(−αt)−exp(−βt)] describing the transient current from ion tracks. The analysis yields a junction time constant Kε₀/qμND that grows slowly with funnel length, shows that carriers initially move by ambipolar diffusion while device fields collapse, and then resume drift once the carrier concentration falls to background levels.

Abstract

The collection of charge from ion tracks can produce logic upset and memory change in high density integrated circuits. It has been experimentally observed that drift conduction usually plays a dominant role when the ion track penetrates a junction. The first charge collection analysis concentrated on the diffusion conduction process. A recent analysis emphasizes drift conduction and describes the "funnel" which produces drift collection from the substrate. The funneling phenomenon has been modelled using two-dimensional computer simulations. It is extremely desirable to develop analytical solutions tp better understand the problem and to provide the basis for modelling the effect in circuit and system analysis computer codes such as SYSCAP. This paper develops an approximate analytic solution expressed as I(t) = Io [exp(-αt) - exp (-ßt)] (1) where Io is approximately the maximum current, 1/β is the collection time constant of the junction, and 1/ß is the time constant for initially establishing the ion track. The junction time constant is shown to be Kεo/qμND, and it increases slowly with funnel length when a funnel is present. The analysis shows that the excess carriers move almost exclusively by ambipolar diffusion for very early times, and that the fields present in semiconductor devices, including p-n junction fields, collapse. Ambipolar diffusion proceeds until the excess carrier concentration is reduced to approximately the background doping density at which time the junction field is restored and the carriers move by drift.