Time-Domain Reflectometry Measurements of Total-Ionizing-Dose Degradation of $n$ MOSFETs

We have performed time-domain reflectometry (TDR) measurements on nMOSFETs before and after irradiation on time scales relevant to MOS radio-frequency response. The decrease in effective impedance of MOSFETs with ionizing radiation exposure is greater for transistors biased in the off-state (all pins grounded) during TDR measurement than for transistors biased in the on-state (VG high and the other terminals grounded). The increased admittance after irradiation is observable primarily in the off-state measurement. The increases in admittance (decreases in effective impedance) with TID correlate closely with decreases in trans-resistance at 0 V. These decreases result from increases in radiation-induced oxide-trap charge and interface-trap charge in the gate and/or shallow-trench isolation oxides.