Stacked RADFETs for increased radiation sensitivity

Hitherto, pMOS Radiation Sensitive Field Effect Transistors (RADFETs) have not been able to detect doses in the milli-rad range, which are required for low dose clinical/personnel applications. This paper reports on further investigation of a design approach, where RADFETs are connected in a stacked sequence so that increased radiation sensitivity is obtained. The radiation sensitivity obtained for 40 stacked RADFETs is approximately 220 times the single RADFET sensitivity. This enables radiation sensitivities in the milli-rad range to be measured. Theoretical equations governing the threshold voltage of a MOS device as a function of bulk-source voltage are used to theoretically evaluate the output voltage of the stacked structure. Measurement and theory are found to agree closely in this analysis. % drift and % fading of the single RADFET, as a function of total radiation induced shift in V_T, is similar to that of the stacked structure