Mathematical Modelling of the Leakage Path Resistance in a Power Bipolar Integrated Circuit Exposed to X-Ray Radiation

The LM2990T-5.0 negative low-dropout voltage regulator was tested under two different input voltage biases and a constant load current in two fields of braking radiation. Using experimental data on output voltage variations and a detailed LTspice simulation model, parasitic leakage resistances were determined, causing this circuit to shut down in most operating conditions. These leakage paths result from establishing a parasitic field-oxide field-effect transistor (FOXFET) between a small-signal NPN transistor's collector and emitter contact in the voltage regulator control circuit. The radiation response of the leakage resistance is complex, reproducing the depletion region, weak inversion, and strong inversion of a MOS capacitor structure. Variations of the leakage resistances resemble the Fermi-Dirac distribution function. Implementing logistic functions didn't provide a satisfactory fit in all three regions. Variations of the leakage resistance in four decades were successfully modelled using the Shah and exponential-linear functions in all the analysed cases.