We calculated neutron induced Single Event Upset (SEU) cross-section as well as the Soft Error Rate (SER) at ground level. For this purpose, we first used an accurate model based on simulation of atmospheric neutron-induced transient currents in a 90-nm drain electrode, through a detailed diffusion model. Then, we performed the same simulations by replacing each transient current by a simple double exponential law model, for which the parameters were set in order to keep the same total charge as for the diffusion model, as well as the same value of maximum current and its corresponding occurrence time. Our results show a little increase of the cross section while using the double exponential law and we established a correlation between the parameters characterizing the double exponential and the diffusion model curves.