The response of a 5- μm -thin silicon on insulator (SOI) 3-D microdosimeter was investigated for single-event upset applications by measuring the linear energy transfer (LET) of different high LET ions. The charge collection characteristics of the device were performed using the ion beam-induced charge collection (IBIC) technique with 3- and 5.5-MeV He²⁺ ions incident on the microdosimeter. The microdosimeter was irradiated with ¹⁶O, ⁵⁶Fe, and ¹²⁴Xe ions and was able to determine the LET within 5% for most configurations apart from ¹²⁴Xe. It was observed that on average, measured LET was 12% lower for 30-MeV/u ¹²⁴Xe ion traversing through different thickness Kapton absorbers in comparison to Geant4 simulations. This discrepancy can be partly attributed to uncertainties in the thickness of the energy degraders and the thickness of the SOI layer of the devices. The effects of overlayer thickness variation are not easily observed for ions with much lower LET as O and Fe. Based on that, it is difficult to make conclusion that the plasma effect is observed for 30-MeV/u¹²⁴Xe ions and further research to be carried out for ion with LET higher than 12 MeV/ μm.