Fully Depleted SOI MOSFET Sensors in Accumulation-mode for Total Dose Measurement

Fully Depleted MOS Silicon-on-Insulator transistors fabricated with a custom process from the Université Catholique de Louvain were irradiated with 1 to 6 MeV X-rays from an Elekta Synergy radiotherapy Linear Accelerator, in order to characterize them as radiation dosimeters. The custom fabrication process only uses Boron implants in the channel region for threshold voltage adjustment, yielding different accumulation-mode P-MOSFETs and inversion-mode N-MOSFETs. The devices were irradiated in incremental steps up to a total ionizing dose of 20 Gy, holding a back-gate bias of 12.4 V to improve the charge buildup in the BOX. After each irradiation step the drain current versus back-gate voltage curve was obtained. Post irradiation response was also measured. The sensitivity to ionizing dose was 165 mV/Gy, which is consistent with the other results for an oxide of this thickness. An interesting difference was found between the post irradiation behaviors of the inversion-mode and accumulation-mode devices. The inversion-mode devices presented a high post-irradiation shift of the current to voltage curves, whereas accumulation-mode devices did not exhibit this large shift. The dependence of the sensitivity and post irradiation response will be evaluated as a function of bias voltage. Also the devices on the front side of the wafer will be evaluated, and the difference between accumulation and inversion mode devices will be investigated