Instead of using encapsulated sensitive volumes to simulate charge collection during single event upset (SEU) mechanism, we use continuous variation of the charge collection in the device. Basically, the collection efficiency is equal to 1 in the core of the sensitive volume and decreases outside. Assuming that the collection efficiency decreases as a power law of the distance, we analytically find that heavy ion SEU cross-section follows a power law of the linear energy transfer (LET). Results are in agreement with a set of various experimental data from 250nm to 28nm. We also show that experimental results allow extracting technological parameters of the sensitive volume as well as the threshold LET, which is a feature of the device.