We calculated Single Event Upset (SEU) cross-section as well as the neutron Soft Error Rate (SER) at ground level for three SRAMs (90 nm, 65 nm an 40 nm). For this purpose, we first investigate the transient current pulse induced at each drain electrode, by using the diffusion model for the transient current pulses. 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 fundamental parameters of the diffusion model such as total charge, maximum value of current and its corresponding occurrence time. Our results show a little systematic increase of the cross section while using the double exponential law. Moreover, we showed that only two parameters of the double exponential law are actually required to investigate Single Event Upsets. Finally, we provided for the 90-nm some analytical expression in order to estimate the distribution of parameters that appear in the double exponential law.